I put a 500 in my 68 ElCamino and have been driving it for many years. It requires a bit of fabrication and some parts substitution, but it is not all that tough. The hardest part is the engine mounts. The Caddy mounts are way forward on the block, so frame "spurs" need to be welded to the front side of the cross member. If you have power brakes, a late model booster will be needed (smaller diameter) to clear the valve cover, and if you have the hide away wiper option, the valve cover will have to be dimpled slightly. If you have air, the suitcase will need to be clearanced and fiberglassed to clear the valve cover. Finally, you will need to fabricate or buy a true rear sump oil pan (the Eldorado pan is a mid-sump). With the stock intake, a 1 inch spacer, and a 3 inch drop base air cleaner, the stock hood fits no problem. If you run an Edelbrock manifold, then I'm not so sure. The Edelbrock does eliminate the possibility of stock air and power steering though. Stock Caddy exhaust manifolds fit the chassis (tight), and BBC headers can be modified to fit the engine with available flange adapter kits. Lots of other info here:
http://www.cadillacpower.com/forum/index.php