X head milled to closed chamber ?

I did it in a restricted racing class with flat top KB pistons ON A 368CUBE 500cfm carb rule. all the intake seats cracked to the spark plug hole in every cylinder, and I was using race gas for 12 to1 or higher. rich carb and never went over 180 degs. and 32 degs timing, they were the x heads,894 69 head. but they did go about 250 miles at full throttle before I found the cracks. might hold up a full season in 1/4 mile racing or on the street with limited fun time. also the rocker oiling hole had to be reworked to keep the oil flowing.