It is not possible to have the proposed layout by modifying a Russian, French (3+3 rows), or a Belgian bass. Some older accordions may have four notes sounding for the seventh chords, but this should not make it impossible to modify such an accordion.
When you modify an ordinary accordion to the proposed layout, the action of the harmony buttons becomes softer. This is because each harmony button now has fewer spring-loaded valve pads to engage.
You need to test that the return springs are not too weak: Depress a major chord button and then the minor chord button to check the spring tension of an individual note. For example, if you first play CEG and then depress CEbG (while still depressing CEG), the note you will be checking is Eb, because that is the only new note to be added. Do this with twelve major/minor couples along a row, in order to check all twelve harmony notes.
It is important that the springs are strong enough to return the buttons to their original positions - even if you lean slightly backwards with your instrument. Once your instrument has been modified, a greater number of duplicate buttons will collapse when you lean backwards while depressing some buttons, and the springs must be strong enough to raise all those buttons again.
The things you will do next will permanently alter your accordion, so don't do this on an expensive accordion, or if you are not 100% sure that the proposed solution is what you really want.
Use a side cutter to cut off all the little pegs (tiny rivets) on the pistons (pistons = the strips of metal to which the buttons are attached) that trigger the unwanted notes. After having cleared the bass strap out of the way and removed the bass cover plate, depress two bass buttons (not chord buttons) to check which pegs to cut off. For example, depress bass buttons C and Bb, and you will see which peg to spare (= E) for the 7th chord button that ordinarily plays CEBb. Figure out which pegs to cut off and perhaps mark these with a dot of paint. Carefully control that the marked pegs are the proper ones to remove - it will be very difficult to correct any mistakes!
Preferably use a smaller side cutter intended for electronics, which has the ability to make a cut that is flush with the flat side of the piston. The freshly cut peg/rivet will most likely leave a small stub that could possibly rub against adjacent levers (= pins attached to the rods). To alleviate this problem, it may be a good idea to flatten the stubs with a pin punch or nail punch (the punch should have a flat tip), and a hammer/anvil. This works well since rivets are made of a soft metal. If a rivet is still rubbing against an adjacent lever, the lever can be gently bent to the side, away from the rivet. Use a slender metal tube, to be threaded on the lever like a sleeve, to accomplish this.
Be aware that when you flatten the stubs, the pistons could sometimes be slightly bent. Make sure they are perfectly straight before you put them back, or else the buttons can get stuck. If that happens, you may have to disassemble the bass machine all over again to fix the problem.
If you are very systematic, you may skip the procedure of marking the pegs and simply remove the pistons and meticulously put them in order and then calculate for each piston what pegs to cut off. To be able to remove the pistons, you may first have to unscrew and lift out the whole assembly of chord buttons and pistons (most notably on some accordions like Hohners), and then pull out some thin metal wires that keep the pistons in place. Once you have done this, remember to not turn the whole assembly upside down or you will have a big "jig-saw puzzle" of expelled (and unique) pistons at your hands! There are 128 pegs (= piston rivets) to cut off on a 96-bass (160 on a 120-bass). Most other accordions, however, require a somewhat different procedure. To find out how it works, search the internet with keywords:
disassembling bass machine george bachich
Although it would be possible to cut off the levers that the pegs push down (instead of cutting off the pegs), there may be some difficulties associated with cutting off the levers. It is difficult to mark the levers to be cut. It could also be relatively difficult to get access with the side cutter for some of the levers (unless the rods can be completely dislodged). The risk is that you accidentally cut off the tip of an adjacent lever if you are not extremely careful. If you cut off the wrong lever, it would be very hard, indeed, to repair the damage.
In the case of Hohner accordions, there are normally also twelve pistons without any buttons attached to them: these actuate the pallets and should be left the way they are. It's important, though, to understand that the pegs on these particular pistons should be on the opposite side of the levers. When the whole button assembly is to be put back in place, it may be easier to do this if the accordion is resting on something soft like a folded blanket, with the buttons facing downwards; that is, the accordion should be turned upside down.
If you start out with a 120-bass, then you can leave out some of the buttons at each end of the button board so that you get a rectangular button layout. The holes thus left open could be considered sound holes, and you could attach a piece of cloth on the inside to prevent dust from getting in.
For an accordion to be modified, the following PDF might be of some help, provided that the mechanical rods in the bass proceed chromatically from F# to F (with the rod of F# being closest to the buttons). In case your accordion's bass rods don't have this setup, it is advised to construct a similar chart before starting to clip off the pegs from the pistons.
The chart shows the Stradella notes associated with each button, but the notes written in black text belong to the new layout, and the associated pegs should thus be spared, while the notes written in white should have their corresponding pegs clipped off. The order in which the note names are written on each button corresponds to the order in which the pegs are arranged on the piston - the higher up the note name is written, the closer to the button the corresponding peg will be.
The depicted layout has 120 buttons; but if you have, let's say, only 96 buttons, you could print out the chart and then cut out the 16 diagonal button rows that you will be needing so that you don't confuse any row on the chart with what you have on your accordion. Of course, you could alternatively just draw some lines that strike through the diagonal rows not present on your bass.
As for register options, the original registers could be kept the way they are, but it is also possible to make some changes. For instance, use thin strips of paper, or masking tape, to block some of the reed block openings if you want to mute certain reeds. If the master switch is switched on, it should be possible to accomplish your favorite register setting by blocking any unwanted reeds.
Do not tighten the screw(s) that hold(s) the reed block in place too much when you put the reed blocks back. Avoid touching the reeds with your fingers as it could affect them negatively. For similar reasons, do not use your mouth to sound a note by blowing directly into the reed block.