Slow cranking speed when warm can be caused by lots of things, but a scored crank bad enough to cause that would have a lifespan of just a few minutes, and the engine would be knocking like a bunch of little guys with sledgehammers were trying to beat their way out of the oil pan. It's much more likely to be poor cooling system performance that causes the pistons to expand and tighten up in the cylinders. Try spinning the engine over with a good battery and with the spark plugs removed. A 52 GMC would have had a 6 volt Positive ground electrical system. Cranking it a little with a 12 volt battery won't hurt the starter as long as you don't overdo it. Cranking for 15-20 seconds at a time without the drag of compression pressure won't stress the starter at all. If doing that builds up good oil pressure, you probably don't have anything to worry about where the crankshaft and bearings are concerned. It's also possible to drop the oil pan and inspect the bearings without doing a complete engine disassembly.
Jerry
