Outlaw,
As for which engine would be better for 400-450 horses, I don't really know, probably the 502.
But,,,,,,as was said in other threads in this forum, there are some '91 and '92 242s out there that came from the factory with 502s. I've seen them for sale on various sites in the past. They are still bringing a pretty good buck (mid to high twenties). Thats what I would do, look for a nice stock boat that came with a 502.
I'll search around for a while, I love boat shopping