67,
I've seen the swap you're thinking about quite a few times. The most common way I've seen it done is with a motor plate to mount the engine. I've seen it done with everything from block-hugger headers to custom headers for exhaust. With a small block chevy the headers from an early Chevy II works with minimal "fixing", but I wouldn't know where to start with any production BBC header that might be a close fit. I had a Caddy 500 in a '87 LX a while back that was a blast to drive! You have to understand that everybody here isn't going to like what you want to do, but it's your car. Do what you want to with it and don't feel like you have to defend it. I don't know how much power you plan to make, but if you run a TH350 tranny, the factory double hump crossmember is nearly a direct fit. Good Luck!