3.6 in A-body

If I was building an early a I would not go the gen 3 hemi route. It’s a great engine and can be bad *** but it sounds like with your current situation fab work needs to be paid for, and a hemi swap needs a bunch of fab, =$$$
I would do a traditional small block and for ease of install look at a 200-4r gm overdrive. They can be built to handle any power level, You can adapt them to a small block, and they are much smaller than a 46re. So floor mods are kept small, although maybe some hammering is necessary. I used a 46re behind my magnum and it’s a good trans but requires the ecu to run it. The 46rh can be used with some Hobbs switches and control is hydraulic, and they’re nice but still huge in comparison. So my early A would be a 9.5:1comp 360-2004r with exhaust manifolds and 2800-3000 converter with steep gears. That would be a fun car and you could drive it anywhere. If I got tired of it I’d add boost. But that’s just me. Happy hot rodding.