I believe they were SR blocks that they used. They pulled your original 400 (or whatever you may have had), and installed a 428 they ordered as a Service Replacement. They gave you a credit for your old engine.

The dealers made out quite well on these deals, because they charged you $600+ for the new engine, then they took your old engine (which typically had very few miles on it), and stored it the service area to be used a warranty replacement for another vehicle.

I recall reading somewhere that there were very few conversion cars made. Car And Driver did the write-up on a 1968 Royal Bobcat GTO with a 428 transplant. From what I understand, there were only a handful of conversion cars made due to the high cost. People opted for the Royal Bobcat package on their existing GTO or Firebird.