Cross-breeding is okay, but the Caddy engines are getting scarce and expensive. Since Ford already makes a world-class torque monster (460), might as well go with it. If you really like that "500" number, it's easy enough to make a 501 out of a 460 with just pistons and rods and machine work. The factory nodular iron crank is plenty strong for the task. It can be offset-ground to make 4.14" srtroke with BBC rod journals.
Everyone knows I'm a Pontiac "bigot", but I do like a certain level of
"purism". I can see crossing brands if your brand of choice (body) doesn't offer an adequate engine family. That's not the case here.
Lastly, I fully understand, a person wants what they want because they want it. That may sound funny, but we tell all our customers: "It's YOUR money. You tell US how to spend it". The GM swap wouldn't be "easy", but not insurmountable, either.