The cam is not limiting you to 6250 your processor is (if still a stock computer).
As far as breaking the lifters you may have too much preload on them. How are you adjusting them? Do you have stud mount or pedestal?
After this is solved if you have the shortblock to handle the rpms you may want at minimum titanium retainers and maybe look into some of the different cam companies newer profiles (kinda like the AFM Hi-Rev stuff) as the ramp rates have improved dramatically over the X cam.
Just curious but why are you trying to spin it over 6k? What is your combo?
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"I'm in the 82nd Airborne and this is as far as the bastards are going!"
93 Cobra, R302 331,canfields,T-76,TKO......sold
87 Coupe, 466, Edelbrock heads,Vic jr,150 shot 10.41@129.5...sold
2006 Honda 450ER .....new toy
Do you own a Torch Red FRC and tremble every time you drive under I75 on Hwy 60 and look in your rearview mirror? If so you are suffering from what is known as Cobraphobia.