I wouldn't go more than 30 over and then find an early 327 crank a set of double hump heads and the cam from a 63 350 hp Corvette along with a small Holley on a good dual plane manifold. And if you keep the slip and slide address the torque converter.
Personally I think the hardened valve seats are unnecessary. In my back yard is a 66 Mustang with early 302 heads that I built in 1979 with over 100 grand on the clock and without hardened valve seats.