You have a GM 90 deg bell housing behind the 151 Iron Duke. A Chev V8 or 4.3 will bolt to the existing setup, I know quite a few guys who have done this. Although the trany is a lighter trany! I agree it should be changed but you could get on the road and go from there.
I wheel with some guys who have 350 chevs with auto's in flatys! This setup has been done and can be a good thing when its done correctly. There are tools out there to deal with driveline issues!
Your motor mounts have to be changed anyway! keep that in mind.
1984 CJ7, 258..HEI..MC2100, T176/D300, D30/M20 lockers, lift, warn winch!
"When in doubt! Gas it out!"