Steve,
There are ways to put manual transmissions behind a Caddy.
http://www.cad500parts.com/catalog/page27.htm. This is the same approach that I am using to put a manual trans behind a Northstar. The flywheels carry the pilot bushing. This approach is also done by most of the VAG cars.
I myself am running a Big Block Olds. It started life in an olds 98 that got totaled. Then spend a while in an 81 half ton Chev pulling a stock car trailer. Now its sitting on a skid awaiting to go into my '48
As for exhaust. You own a welder, make your own headers. That is what I am doing. The good news is that you dont have siamesed exhaust ports, so you can start with a set of BBC headers and change the flanges.
The trans I have is a TH400, however seriously looking at a TKO-500 or even a 4L80e with a trans adapter and stand-alone. Most likely, to get the truck rolling, I will stick with the TH400 for now.
