as cast, the 69/70 351W heads (C9 AND DO castings) came with slightly larger valves and intake ports than the standard 289/302 head, making it a natural bolt on horsepower increase. There is enough room to further open up the heads to 2.02 intake and 1.60 exhaust valves and plenty of "meat" for a good porting job to really wake them up. Before the big influx of aftermarket pieces for the small block Ford, the early Windsor heads were the way to go, unless you opted for the "Clevor" approach using Cleveland style heads.....
with all the head offerings these days, especially from the aluminum side of the house, the Windsor heads are not as popular as they once were for obvious reasons. In fact, there is a very nice used set on E(b)ay at the present that has already been modified with the larger valves and comes complete including guide plates and roller style rockers for a buy it now price of 200 bucks!! Still, if you are trying to really wake up the potential of a small block Ford, there are better heads out there these days...
randy