I agree, if you have the choice, use the later heads. I did a cost comparision between reworking a set of 70 something 302 heads, with the small, 54 cc chambers, that I have, etc, and buying used GT40s. After including installing bigger valves and seats, milling, valve job, springs, new rockers and pushrods, (and, or guideplates), modification for studs and studs, a set of iron Gt40s was cheaper! About $700, with springs, milliing, and valve job, on the car!
The GT40s bolt on with stock rocker stuff, and run easily to 6k.