My first guess is not enough valve spring pressure on the seat.
What I jst can not understand that I see so often now days is all of you who are beating your Big Block Chevys to death alsmost by operating them in the danger zone window. 6,600 to 6,900 rpm is the worst possible rpm to be running the Big Block Chevy. Some people do it and get away with it, and some don't. Reving to a higher rpm is actually much easier on that engine if you are using solid lift cams.
For some reason with hydraulic cams that rpm is fine. The valvetrain harmonics when using siolid lifts of flat tappet or rollers at that rpm is horrendous and is much easier when you go up to even 7,500 rpm.
I have never ran a BB Chevy with solid lift at less than 7.500 rpm for shifting rpm even with the old oval port heads.
Even the bigger engines called Mountain Motors are run much higher. There was one that was big using a 5.750 stroke that did end up shifting at 7,200 to 7,400 rpm depending on the track conditions. ^32 's are being shifted on all motor engines at 8,00 to 8,400 rpm. For the life of me I do not understand all of the newer guys who keep the rpm down on those BB engines. They love rpm.
My siggestion is to watch your valve spring pressures both seated and open and consider your spring set up as a critical part of your engine. Never set them up with more than .080 from coil bind. Off the top of my head with the cam specs you supplied I would be looking at a minimum of 260 lbs on the seat.
Ed