Yes I have tried all of the caps. Yes I am putting them back in the right spot, not only are the numbered but I also took pictures of it when it came back from the machine shop.
To get the measurement down inside the block, I used all thread and some long nuts. All thread about an inch shorter than I needed. Backed the nuts off the all thread until it was tight (not crazy tight) going in and out of the block. Locked the nuts with secondary nuts. Took it out and measured it. I know it might be off a little, but certainly not .020 of an inch
A micrometer works on the same principle, but there is a difference in precision.
I don't know if you have said how you tried to install the caps, pulling them on with bolts or if you were just using hand pressure which will not work unless they are too loose.
As others have said, if there was actually a .02" thousandths difference there is no way they would go on the block without a tremendous amount of force, that's a huge press fit. If one were to somehow use enough force to get them on, substantial damage would be inevitable, might even split the main bores.
If the caps were actually installed at the machine shop there will be marks on the cap register surfaces and if they were honed on your block there will be honing marks that match between the caps and the block. Each one will have a slightly unique pattern and with a good eye and careful inspection many people could easily tell if the caps matched the block and even know which cap goes where, even without stamped numbers.
Assuming the caps were properly installed before, it's just a matter of using enough force to pull them on with the bolts or studs. Alternate back and forth to keep them reasonably even.
It is possible to ding up the registering surfaces with improper installation techniques, which could increase the pressure required substantially. So it's not a bad idea to closely inspect the mating surfaces for any damage and dress the high spots if needed.
