Oil clearances and a surprise

Pistons on backwards?

As long as there isn’t much variation in piston or bore sizing, you can just swap the pistons/rods side to side, if you want the rod chamfer, piston notch orientation “correct”.

Having them on backwards is an old racing trick anyway.
Put 1 in 2, 2 in 1, etc.
Supposedly reduces the skirt loading from the pin offset(which is only there to reduce noise at cold start up).

If the issue is race type pistons and the valve notches....... that’s a different story.

What pistons are you working with?