Seeking 500 cubes

I currently have a 368 in an '80 fleetwood. I would like to go ahead and get a 500. What years are good? Would it be better to get a donor car with one and rebuild, buy a core and have it built, or wait and purchase a fresh long block. I would like to keep it mild. Will my accessory brackets work on a 500? Can I use the same radiator? Any other tips or suggestions will be appreciated.

Nevermind, I found a '76 500bb, 120cc heads, and an intake for $300. I am happy with my purchase. Looks like I will sit on it and build it when funds permit.