Some updates on progress on the engine! And some history... I got 2 engine blocks with the car, the serial numbers identified them as from a GT6 and a GT6+. Which is odd because the tub is from a mk3.
The GT6+ motor seems to be hosed... I was able to get 3 pistons out so far, but the last 3 are pretty solid.

The gt6+ engine on the stand with all the pistons in

We managed to get 2 of the pistons out, but pushed the other 4 down to try and loosen them. After soaking in ATF and acetone for a week, I got 1 more out.
So I decided to put the mk1 engine up on the stand and pull it apart, as it was the last engine in the car and I knew it turned over. A short time later all 6 pistons are out!

But the pully on the front of the crank is refusing to come off. Go figure...