I think we pretty much bashed this idea into few options for you.
The limiting factor is the old Dual Range Hydro-Matic transmission. It only bolts to pre-1964 engines. The new 64 up BOP-C bolt pattern on the block doesn't accept the older bell-housing. The Chevy bolt pattern is different again and while it possible to purchase Chevy/BOP-C adapters; the only source of 64 and up BOP-C or Chevy to Hydro-Matic transmission adapters has long ago dried up.
This limits you to using a pre-64 Caddy engine, there's worse things in life, if you want to retain the DR Hydro-Matic. Or:
1, You get lucky and find an old B&M adapter Chev to DR Hydro-Matic at a swap meet, Hemmings, e-bay, something like those places. Or you stumble into a 64-up adapter for BOP-C which would let you use the more modern Caddy V-8s.
2, You change to a TH-350/400, 700R4/4L60, 4L60E/4L80E gearbox which will let you run any modern GM V8. Once again this is simpler as Chevy to Chevy; but Chevy can be commercially adapted to 64-up BOP-C and some of these came with both bolt patterns.
Lastly you need to realize that the Cad's value to collectors goes way down with any non numbers matching engine and a non-original type transmission. So I guess before breaking the numbers matching stream you have with the 390, it would have to be a basket case before I go down that road.
If you don't care about numbers matching and want to be in a world where parts are available at reasonable prices, my modern response would be a BBC with a TH-400 or if wanting more recent technology, still a BBC with a 4L80E. The latter gear box will give an overdrive 4th combined with a lock up converter for really efficient, slow RPM cruising. Plus it has a really deep low, like your DR Hydro-Matic, to launch this heavy car. If you went for a fuel injected BBC this would be a super set up.
You could do a 350 or even a 383, but from where my experience has been, when you talk a car that's as heavy as a modern 3/4 ton pick up, you'll be happier, longer with the engine and transmission from the 5000 pound truck. It's not that you can't get power out of a SBC, but asking it to that and move 5000 pounds of vehicle really reduces its life span. Besides a 350 just wouldn't have the torque to launch the Caddy, espacailly with a TH-350 transmission. which has almost a ratio higher low gear and no OD. Getting the Caddy launched takes a lot of torque, this really takes some cubic inches from the engine where a long stroke is better at torque on the bottom end where it's needed with this rig; and it takes some steep gears to help get it launched. I think an SBC with a TH-350/700R4/4L60/4L60E is in over their structural head. I' mean they'll do, but it just won't last.
Last edited by oldbogie; 05-11-2007 at 04:37 PM.