To the best of my knowledge there are actually no engine casting numbers on late model 302 blocks, the early blocks have casting numbers as described behind the starter, the number refered to on the machined surface near the distributor is acually an assembly date code which usually goes away after the first decking of the block (which is an easy way to tell if the block has had been to the machine shop). Late model blocks, heads, etc. as far as I know get ink stamped numbers that go away as soon as they are painted or just with them surface rusting over time. Hope this helps.
Chris