lux hauler is right, it should be pretty simple. The 425 and 500 blocks are the same size, as is a 472. You should check out the following 2 pages:
http://members.tripod.com/~CadillacMan/79cadpage.html http://www.mindspring.com/~dburden/engine_swap.html
If you start looking for a 500, know that they were used in 70-76 in FWD (I think) Eldorados, and I'm sure they were used in all other (RWD) models in 75 and 76. An all stock 76 500cid has been dynoed at 300hp and over 500ft.lbs. torque. So you'll probably be happy. There are also performance parts available from 2 major suppliers:
MTS -
http://www.500cid.com/
Cad Company/Flashcraft/Cadillac motorsports division -
http://cad500parts.com/