Brad - I tried an underdrive pulley on my old 63 4 door w the 512" and I did not have success with cooling. It had a stock BB radiator, flex fan w/shroud and it could not keep up. I switched back to stock pulley and cooling issues went away. Matter of fact, I realized the cooling issues when I was driving to the track. Part way there I pulled over to let it cool down and then limped it back home. I never did get to find out what it was worth on the timeslip. It might have worked with a better cooling system, who knows.