That 406 I built was a flat tappet 560 and 580 with 256 and 262 with almost identical combo as yours and it was just awesome! His car is 3200 with driver with 4:30s and a glide with a 5000 stall. That was with pump gas! Dont worry, unless you live in Denver and every other thing on the car is a mess, it will run in the 10 sec easily. Ours ran high 10:60s and high 10:70s in the worst heat and humidity in Georgia. We are in North Georgia so the elevation is about 1300ft. It made me rethink my BBC combinations......like what am I doing with them! Now we had our heads ported specifically for this combo by Jon Kasses head porter so we did spend money there but the rest was a GM block and Eagle crank and Rods.