I'd also say that it depends on what application he is wanting, because yeah, I wou ld probably go with a 460 as well myself, but thats because I just do hauling and all that such. But he doesn't ever make it clear what he is building the truck for. For all we know he wants to make a mud runner out of it, in which case, putting in a 460 wouldn't be necessary at all, and the 390 would work perfectly fine.
So I can better answer the question as can all of you if he would be kind enough to reply, telling us what primary application he is going for.

Thanks
-Daniel