Its super easy to notice the BIG 302 or 351 numbers on the heads..
The 351 will have letters "WF" which will be Windsor foundry.
Now the difference depending on the castings is port size, chamber size and valve size.
You have to be careful with that.
If your rocker arms are self adjustable (rail style)
and its better to go with the 351 heads.... You sacrifice like 4 ccs of chamber but gain a lot of port space and more valve....
Its all better