How much bigger are the GM metric calipers over the Ford units? It maybe a matter of volume and the present master cylinder can not supply enough. You may need to upgrade to a bigger bore cylinder to provide the needed volume. Can you do a quick pump of the peddle and get it to hold solid up higher, or does the peddle slowly go to the floor with pressure on it. If it slowly drops, its probably air still in the system.
Sometimes you need to tap on the caliper to get the air out of them, and sometimes you need someone inside to pump up the peddle four or five times in quick secession, then open the bleeder while still holding down on the peddle.
Joe
