I guess you need to define "build for HP".
He looking for 200HP, 300HP, 400HP????
For 200HP the six will do it fine. If you're looking for much more than that the V8 is the only way to go. But you're now talking $$$$$ to get in the 300HP area.
Yup you'll need the 360 radiator with the swap also but prolly even a bigger after market one(3 or 4-core) depending again on the HP goal. The engine is pretty much a bolt in with new motor mounts but you'll still need to deal with mating the rest of the drive train up.
Just curious but if he's worried about cutting his gas mileage why's he building for HP? He's prolly gonna lose 25% mpg's just by dropping in a "stock" 360
let alone a built one.
If he does use your 360 and he wants to get rid of the 258 drop me a line direct. I'm looking for a complete one that's rebuildable.
Preferably a 70's vintage but would take an 80's too.