it should still work thenbecause the 283 and the 350 have the same outside block dimensions and motor mouting locations/bolt patterns. the turbo 400 trans mount is probably the same as well. some had longer or shorter tailshaft housings and the same for the 700r4, but the mounts are probably in the same location. a trans guy would probably know or you could easily find that out on a google search. this is from the pirate 4x4 site. it shows the gear ratios.
TH400, 3 speed 1st 2.48, 2nd 1.48, 3rd 1.00
TH350, 3 speed 1st 2.52, 2nd 1.52, 3rd 1.00
4L80, 4 speed, 1st 2.48, 2nd 1.48, 3rd 1.00, 4th .75
700r4, 4 speed, 1st 3.06, 2nd 1.63, 3rd 1.00, 4th .70
The 700r4 is the 4speed version of the TH350
The 4L80 is the 4speed version of the TH400
this is from jalopy journal, just scroll down to the diagram in post 16
http://www.jalopyjournal.com/forum/t...-700r4.500280/
this is from the corvette forum and has some tips and dimensions etc
https://www.corvetteforum.com/forums...rans-swap.html
anyway, that should get you going. a used crossmember set up shouldbe relatively cheap at pick n pull if you can find one, or craigslist or just post an ad in classifieds on this site. they are pretty strong and tie the two rails together but unbolt if you need to. I think.
just some thoughts