Yes, you can put 351W heads on a 302. You won't gain anything though unless you went to an older casting...like early 70's. Early castings were already cut for slightly larger valves. Starting in I believe 73 they had the same size valves as the 302, but still had larger ports. Late in the 70's they had the same ports and valves as a 302 but with larger combustion chambers to drop compression for emissions reasons. Starting in 87 they are the same heads, both use the E7TE castings. One thing that is different between all years, is the head bolt size, the 302 uses a 7/16" head bold, while the 351 head uses a 1/2" head bolt. This doesn't really matter though since you can get head bolts that have a washer face on them for this application. Almost all aftermarket heads have 1/2" head bolt holes in them. You can get these bolts from Ford or Edelbrock, probably ARP also.

-Darren