Whimpy, If I'm reading the block casting number correctly - 3850817 - that block was used in both 230 and 250 engines, as 3B suggested. The 250 was introduced in '66 with the same bore dimension as the 230, but longer stroke. The date/ date code may help you ID which engine you have, specifically, if it is original to the truck.

Doug

1947 2nd-Series GMC FC152 3/4-ton
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1953 Chevy Bel Air Sport Coupe Powerglide