Too Tight Rebuild
What about the end gap of your rings.
Good point Snake.
Let's start with the crank, once that is verified, go to the pistons, rings & rods.
After the crank is in and turns freely, then check your ring gap for the rings and then install the rings on the pistons and start putting in the pistons and check bearing clearances with plasticgauge. See how many you can get in before it gets tight. It should get a little tighter with each one, if it gets real tight, look closely at that piston/rod/ring that made it get tight.
If you find one main bearing or rod bearing clamping makes it get tighter, also check behind the bearing shell and make sure that no dirt got behind the bearing shell. That could take up your clearance and make it tight.