If it's a four door then putting in a Caddy engine would make a nice low budget car.
We all have a car or two that we never want to sell but sometimes unexpected events make cash a higher priority. If you're willing to accept the lower resale value and make fun the priority go for it.
I would consider the Caddy engine only if It ran good and didn't need a rebuild. If I had to build up a big block engine I would spend my time and money on a Chevy engine.
I have a Caddilac shop manual for 1985 Caddilacs. It covers rebuilding the 4100 engine in great detail. If you can find one for the 472 or 500 engines it'll have all the info you need to rebuild one. Shop manuals sell on ebay for about $25. You may have to keep checking ebay for a while to find one. Swap meets are another source for these books.
The local GM dealership has Manuals going back to the sixties. When I worked for them they let me borrow any that I needed. They would let anyone look through them and even make copies of the pages that you want.
If you don't know anybody that works at a dealership you could take your car in for an oil change and ask the service manager if you can look at a shop manual while you wait. They treat customers better than a stranger coming in to bother them.