OK. The TRW pistons you have were designed to be used with rectangle port closed chamberGM heads
These heads are not the same in the combustion
chamber as the oval port. Tho, to the casual glance they look alike. The rectangle port heads use a chamber that is releived more around the perimiter. Making the chamber just a bit larger across the distance of the valve faces. The oval port head will hit the pistons on the exhaust side at the deck and outer edge of the dome.