I really think that dropping a 250 in is your best bet.
It probably wouldn't take any modifications at all.
Makes it a lot easier to start and there's probably 20 guys here that'll trade a running 250 engine for a 500 that needs transmission work.
Just think about how easy it would be to work on...


Other than that, dropping the compression would be the first thing to do in de-tuning a zilla, followed by a smaller carb.