I can't remember what was allowed in terms of Cleveland heads on a Windsor block. I do remember something about a water passage needing to be made. As far as the Windsor heads, there was a 4-barrel high compression 351 made in '69 only. It would be ideal, but good luck finding a set. Perhaps 289 HiPo stuff? I've also heard that you can machine the Windsor head to use Chevy valves (seriously!) to your advantage. I don't know what to say about GT-40 stuff. Perhaps a Mustang forum could give you the skinny (hey, what can you say about Chevelle people giving you advice other than they're a class act?). Even basic 351 Windsor 2-barrel heads would be an improvment over the regular 302 heads, and they are way easier to find.
The 5.0 started using a 4-barrel around '84 through '86, so there may also be some other avenues there.
Those Ford guys have it rough. FoMoCo simply does not allow for obsolete part numbers from a previous decade.