Radiators: Is 22" enough for 400 HP?

Good idea. Since you need one anyway, you might as well go a bit bigger.
Be sure to take a snap shot and report back with who you went with. I'll be approaching this possible problem myself right soon.

This is good to read even more so considering your location vs. mine. I'm doing a 360 for the wife's '67 Cuda.

Yep, been taking that into consideration too.