Where is the casting numbers at on the GMC inline6? I found a 52 or 53 duce and a half with a GMC 6 in it And the oil filter covers up the serial number. Whats the chance that it is a 302?

Chances are very good that it is a 302.
The is a flat pad cast on the block just behind the distributor. The first three numbers stamped on the pad is the engine size.

Also check out the casting #'s guideline on this site and it should help you with other numbers you can use to determine if it is a 302 or not.

Thanks for the reply Arlyn. The pad you are talking about is half covered by the oil filter. The oil filter is behind the distributor on this motor. But I found some pics on the net just now. And it appears the casting number is on the drivers side middle lower part of the block.