Radiators: Is 22" enough for 400 HP?

It is positive only as much as there is airflow across or around the extra area. Almost all of the heat from a rad is from the fins and tubes, and they for sure need air flow as their cooling surfaces are >95% within the interior of the core, not on the exterior surface. The only other way for heat to get out is by radiation, and on a shiney AL rad surface that is very low! (BTW, the old rads were painted for protection and flat black is an excellent radiation surface....)

Not trying to be contrary at all but 4" is an 18% cooling increase... that ain't hay! Just some thoughts that might help you:
1) If cutting the core support is not good, then if there was room, then maybe space the rad 3/4" or more back from the core support, and seal up the edges and top and bottom between rad and core support. It would be kinda like a shroud in reverse with reasonable turns for the air to enter the 2 extra inches of core on each side.
2) If that is not possible, event taking a 1" to 1.5" hole saw and putting extra holes along the edges of the rad opening in the core support may help a lot.