All in all the least intrusive swap would be a Clark 207vo or smaller if you can find one. The older overdrives are usually hard to near impossible to come by. The 207 should have a smaller retainer that I believe would work in your bellhousing, and there was one on ebay for a while..
Most of the np540 transmissions that I have come across had a different bolt pattern, larger and longer input shaft, and are 1:1 an od NP 540/541 trans is scarce today (like chicken teeth lol). Something to look into would be fitting the gear vendors over drive for the older Dodge NP435 to a 540 since they shared the size and spline on some of the output shafts like my 542GL.
Like the NP trans most of the larger Clark trans had the large 1 1/2 10 spline shaft, and the OD versions of those are also hard to come by.. not to mention parts are getting harder to find as well as support info.
I think you would be happier with a NV4500 trans for your swap. They are plentiful, parts can easily be found, shift nice, and have a higher O/D ratio than their older counterparts (.75) which would be nice with your current gear ratio. once again you are not going to have a direct fit swap, none of the NV4500 transmissions share a bolt pattern with any of the 4 speeds. 93 to 95 GM transmissions have the right input bearing retainer pilot diameter of 5.125" but I can't rember if the input shaft is long enough for an adapter plate to match it to your bell housing or not. Gotta do some measuring to see. Looking foward to see what you do!
