Radiators: Is 22" enough for 400 HP?

The cooling required depends on driving conditions. If you stay down in the LA Basin flat-lands, you should be fine. Manufacturer's test new cars for worst-case, which many use the I-5 N climb out of the LA Basin at 70 mph when 115 F at Lake Hughes as the litmus test. In the first warm Spring days, you see a vehicle pulled over every 1/2 mi w/ hood open, usually gomers who didn't check their coolant level all winter. Ditto for the I-15 E climb. Many fairly new cars in the bunch, and those aluminum heads warp easy.