i have a bbc chevy th400 in my 37 gmc right now. i was able to do this back in high school, so it's not that bad. i still have factory frame, front axle. you have to deal with the steering box, i suggest the mid 70's chevy truck box on the outside of the frame up front. the other thing is exhaust manifolds are too thick, i used some generic headers, set the height of the engine to frame based on the headers. i used speedway motors front cradle mount that bolts to the block on each side of the balancer.
the firewall had to be clearanced on the drivers side for the valve cover.
throttle had to be modified, i used a cable
stock brake pedal had to be moved, it wouldn't clear the engine at all, not even close.
removed trans crossmember, replaced with another to fit new trans.
i was able to use the original radiator, but was not pulling any thing. mine was also a long water pump late style, i used a early 80's GM electric fan, it fit the rad perfectly.
i also have a 5.9 cummins, they are longer than the factory 6 cyl, i think that they are app 33-34 " long.
caddy motors are wider than a bbc, get the same mileage, less parts availability.
sbc is essentially the same work as a bbc, potential to get better mileage when empty, but smae or worse pulling.
all that said, look at the newer LS series engines. i bought a 2000 5.3L for $800 that is going in my 68. all said and done the cost is the same as a carbed sbc, bbc, but you have some advantages over them. these don't have a distributor to work around, aluminum heads, nice reuseable gaskets, etc.
good luck
