My goose supposedly came to the third owner with a 351W in it....as #2 had blown up the 302 somehow.... he couldn't recall the exact details, but he remembered that it had a 351W when he sold it. Can't find #3 yet, but #4 got it with the ill-fitting Boss.....
From the looks, of the C8FE heads, I would think that the 351W castings could be taken very close to that size shown. The ports in my other 69-351W are very near that size....matching a set of old Cragar headers I have somewhere! Switched to shorties and had to "round" out the ports a bit more....! LOTS of meat in the 351W heads... ...stop when you hit water!!!
Not much to hit grinding around the sides and top, just stay out of the water jacket on that bottom area of the lower curve into the valve pocket area. "Smooth and blend..." Don't necessarily make it bigger everywhere.....If you can find a set of the original aftermarket headers that Hall sold back when, they have the larger tubes in them. The originals were very necked down and restrictive....not to mention made of tin foil it would seem. The standard 302 heads would respond very well to gasket matching to the new headers. Seems the original headers were made to match the Ford ports as cast! There were some odd openings in those headers that I looked at!