If you could say cut the log manifolds outside off and attach the GT manifolds to the logs to extend their volume and make them a 2.5 outlet? I did it once on a pair and they worked well, but man they were fugly and it wasn't worth the heating brazing, heating brazing... Too bad that there can't be a set of new "factory replacement" GT manifolds cast. Mine was like a 1.5 inch spacer, it looked awful, but it was a pulling truck that had to be "Stock" and it passed tech being called a spacer. It did more than one full pulls before the welds broke.
Why not improve the GT manifold and get Ford to slap a number on it? How a about a 410 based GT? maximum .040 over.