B body A833 with a gear vendors on the back
expensive
gets you your overdrive without cutting the cross member
need to address stick position problems tailshaft problems and potentially clearance the tunnel further back with a big hammer
i only know this cos someone here did it. read it all he had numerous trials
New build A833 18 spline with Gear Vendors OD.
the only tremec i have seen that you can fit to an A without cutting the crossmember out was the never very strong TR 3550 box provided by keisler
it had the mid shift tower machined flat to make space for the cross member and worked with a standard bell and 1/4 inch steel adapter.
but still needed a new hole for the shifter and the front of the tunnel slit expanded and patched
it would not be good behind your motor in the same way a t5 would not be good behind your motor tko or tkx might be but both involve surgery as i belive you have already recognised in your post
Dave