400 Cylinder Walls Breaking ???

built a low deck 470 motor for my dart. first go around used 516 big valve heads ported by me. ran 6.40s with no issues. first freshen up scored a set of victor heads cnc by hughes. put a small roler in it .660 lift . first pass 6.24 was very happy. started upping shift points and kept going faster. with 516 heads 6200 was fastest. tried 6400 then 6600 at 6800 got 6.13. was hoping to see a 5.99 or better with 7000 or 7200 shift. next weekend car slowed over a tenth, trouble shoot and yep #8 split cylinder wall. sleeved that cylinder and put it back together. have not raced since. know john very well, motor was based on his combo. was leary of putting too much power through it for the very reasons stated. john was pulling mains out, i split a cylinder, go figure.