many, but not all f the old SBC heads that had a "X" at the end of the casting number, had a slightly larger intake port.
the 487(X) heads came on 350's and 400's. the 400's had 1.94x 1.60" valves and extra "steam holes" between the cylinders on the decks. The 487's that cam on 350 could have 1.94x 1.50" valves or 2.02x 1.60' valves. the chamber size varies from 76cc to 80cc. They only way to tell if the two heads you have, have the exact same chamber size is the cc them. Ya can't tell by looking at them. But if they have different valve sizes say 1.94x 1.50" in one had and 1.94x 1.60" in another that might be a clue. You could equalize and eliminate any difference in port size by porting the heads. and replacing the valves so they match if that is important to you.