Engine very very tight after reaching normal operating temp

89 Max 2.0 truck, having a major issue after installing a rebuilt short block.
long story short. When cold, engine turns over normal and runs just fine.
THE BAD. After engine reachs normal operating temp. Upon turning the engine off, the motor is to tight to turn over, its is basicly locked.
I even tryed to put a wrench and a pry bar on the crank. Im a big fellow and I cant budge it.
let it cool for a hour or so. Cranks and runs fine.
What the heck is the problem?


** here is what I know, I have drove it 75 miles, I put a real oil preasure gauge in and oil preasure is within spec, no overheating, not a starter issue, I removed the oil filter and cut the outer skin off which revealed no metal at all **

Now, What the heck is wrong. Im at a loss !