I guess I really don't understand the question. Two 21's with different stock dimensions & you want to shoot International Trap?

Simple, assuming you are going to keep the guns original, buy the one that comes closest to fitting you or better yet buy a non original vent rib 30" Or 32" gun & alter it as needed. Nothing like shooting a gun that fits, nothing worse than trying to shoot with one that doesn't. The bbl length is going to be a matter of personal preference.