Even on engines without shims between the block and cap I can make a case for not doing this modification.
On a high mileage engine it is not uncommon for the main bearings to see wear. This will show up as low oil pressure on the gauge.
Through the years I have worked on many of these engines. There are two fixes for this, both require pulling the main caps.
Most expensive is to replace the bearings. Second is cheap and easy, (if the inserts are not too bad), and most people do not even know about this process. It was common practice in the old days to shim between the insert and the cap. This forces the bearing closer to the journal. In the old days they sold tapered shims just for this. I even do this on fresh rebuilds if the clearance is not right where I want it.
