The 280 spec sheet shows a .520" lift, so be sure the valve guide height and the oil seals are correct. On some high-lift cams, I've had to shorten the top of the valve guide a little to prevent things from getting together at max lift. Always assume everything is wrong, until you prove it's right. When you get away from stock parts, you become the engineer, and it's up to you to make everything work together.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!