in school they tell you to keep the main caps in order but never talked about bring caps from another engine ...they assumed you are working on a late model engine that came to you complete.
Once you take a cap (for a rod or main bearing) off of the rod and put into anywhere else other than the original piece that it was machined with, YOU HAVE TO REBORE/ALIGNBORE IT!
You cannot take a main bearing cap from #2 position and put it in #1, #3, or #4 position on the block, It has to go back on #2 position. Every cap must be put back to the original position and orientation (anchor slots lined up butt together) for it to function properly. I've seen cases where mixing up the order of the main bearing caps won't allow the crank to turn when the main bearing caps are tightened down.
If you don't believe it, just try it next time you put an engine together and see how it works. Switch positions of a couple of main caps and torque them down and see if the crank turns and how free it turns without pistons installed. Then put it back together correctly and see if the crank turns any better.... It will make a difference.