OldIHC

Slowly working on rebuilding my 233 engine. Measuring the Main and Rod journals, they measure to the correct specs (standard size). They also appear to be in good shape.

I know the trucks history pretty well and don't think it was ever driven many miles.

I have limited engine rebuilding experience and am wondering if this is common for a vehicles crankshaft of this age to still be within spec. Thanks in advance.

Yes, it isn't uncommon. The crank is hard and bearings soft, so the bearing is what wears (at least at first).
It is quite common for heavy diesels to go many overhauls with just replacing bearings with standard replacements.
The metallurgy of crank can make a difference, in automobiles the cranks are less critcal and less effort has been put into them from the factory, still many are re ground because people think that is required for an "overhaul".
The important thing is to not let bearing wear get to the point that the crank starts to wear also.
Again in heavy diesels, 700,000- 800,000 miles and you better roll in new bearings. I know a guy who did it every 175,000-200,000 miles (which was more common in the "old days".