That was standard practice back when these trucks were daily drivers. The individual shims are about .001" thick, or maybe a little less and there's usually several of them stacked up together. Peel off a layer or two and snug the cap up to see if the crankshaft gets hard to turn. The ideal situation will be when the crank can be turned with a firm pull on the bottom of the flywheel, but not locked up. Check and adjust the main bearings one at a time with the others loosened up slightly, so the ones you've already adjusted don't interfere with getting the one you're working with right, then retorque all the bearings. Rookies use plastigauge to check for the ideal .0015" oil clearance- - - -us oldtimers use a narrow piece of .0015" shim stock between the bottom bearing shell and the crankshaft and add or remove shims between the main caps and the block to get the right drag. It's possible to split the difference with one more piece of shim under the main cap than the other side. The front main cap has two 5/16" bolts that go into the bottom of the timing cover from the crankcase side.

Connecting rod clearance is adjusted the same way.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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