If you are using older heads, the 351Ws have more intake bolt holes. These are there solely for your amusement and you can ignore them. Weiand seems to be the only aftermarket company that ever bothered including provision for these extra bolts holes, nobody else ever bothered. Nor should they bother you. The later heads (1975 and newer IIRC) didn't have extra bolt holes.
You'll have water passages in the ends of the heads that are either "square" or "L-shaped". On some gasket kits they include both types. Others just include the square types. The square ones work either way, the L kind can be trimmed a bit if needed and be made to macth a square opening. You can ignore the port of the intake, just how the ones on the heads are made is important.
If you don't want to use headbolt shims you can buy "stepped" headbolts from ARP that are purpose made to bolt 351W heads onto 289/302's. Shims are cheaper but ARP bolts are top quality. Either way works though.
I have some specs around here somewhere, but right off the top of my head you would be gaining very little and likely losing some compression. If you were stuck with having to use premium gas before you might be able to get away with the cheap stuff after swapping them on. Given today's gas prices this may well be no bad thing. The lower compression costs a bit of horsepower though. The 75 heads will also have a "thermactor bump" in the exhaust ports. A nasty restriction in an already cramped exhaust port. I don't belive 68 302 heads had such. You can of course grind the bumps out and I highly recommend doing so. IIRC, the 75 heads have slightly larger valves than the 302's so they have a bit more potential. And with a good port and polish job they start looking a LOT better.
I have a set of 1975 351W heads. They have the same size ports anf valves as the earlier 351W heads, contrary to what I've read. The smog bump is FRIGGIN huge! It takes up most of the exhaust port and has to be 3x the size of the ones on my E7s. Whatever you do, grind out the smog bumps! BTW, that intake sucks...get a performerRPM or wieland stealth.
The valves didn't shrink to 302 sizes until 1978. You are correct about the bumps. I cleaned up a set of 1970 heads until I was absolutely sick of grinding. Off and on, it must have taken me a 4 or 5 weekends to shape them up to my satisfaction. I recently built another 351W and happened to come up with a good set of E7TE's. I was amazed. The same sort of minor port, polish and cleanup all done in one weekend. Had I wanted, I bet I could have done them in a day.
That'd be expensive as **** to make it all fit, and at the end of the day, you can still pick up a very nice 347 kit for $700. That said, that's how 302 strokers were made back when...351C crank and 2.3 rods with SOMETHING wierd for pistons IIRC. http://www.bacomatic.org/~dw/302strok/crank.htm
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