Heads bolt right up, but the bolt holes are 1/2" on 351, 7/16" on 302, you can buy collars to use 7/16" bolts with the 351 heads. Most 302 & 351 heads are the same after 78 (I think) so there won't be any cfm gains. The 351 intake will not fit the 302 at all, it's too wide.
Look for 351W heads with casting numbers C9OE or D0OE. For 351W to 302 swaps, these heads had the best valve/runner combos. Depending on the year and pistons in your 302, you could loose up to a point of compression, but you can mill them to get it back, and a decent porting job on these heads really changes things from the stock 302 heads! I think Ford Motorsport (or whatever its called today) still sells the stepped bolts for this. Use 351W head gaskets. There may be some question about the length of pushrods to use. Stay with the 302 rods and you should be okay. Someone here will tell you how much to mill them. I don't remember, but mill the intake sides of the heads as well for a proper fit.
Stay with the 302 manifold. 351W engines are taller then 302's so the manifolds are a bit wider and won't swap over.
There is an on-going debate about the cost effectiveness of the ported early heads vs. aftermarket versions such as Roush 180's. For all pracical purposes, the early 351W heads share the same deficient exhaust port as any production small-block Ford head.
I agree that the Roush heads will easily outflow the early 351W heads, but I can't agree with the cost compairison. You can get a complete set of early 351W heads for as little as $50 if you look around (try Ebay), and a decent porting kit for even less. If you know how to port, there are a few sites out there that will help you handle the exhaust runners.
Will you get the same numbers as the Roush heads? Not likely, but it will easily outflow the stock 302 heads and is pretty cheap to do. After all, we're talking bargain basement performance here.