The date code for the block is below the oil filter adapter mounting pad. It's a number/letter/number(s) sequence and sometimes hard to read/see. That's a pretty common casting number also. C6ME-A was on 352,330hd,360,361,390,391 and the 428 blocks. What are the casting numbers off the crank and rods? And as Primer mentioned the easy way is to measure the bore and stroke. Just keep in mind the overbore and some smaller bore blocks could be opened to at least std 390 bore. A 4.05 cyl bore and 3.78 crank stroke is what your looking for a 390. G.