The running in process is purely to work-harden the cast-iron cam and as it only spins half crank speed, the revs must be held @ 2500rpm to prevent the lobes "knocking" slowly against the lifters. Once the cam has been runn in you CAN fit other lifters providing they are new. Back in the day i had an original RST with 68k on the clock which was sweet as a nut, put some lifters in for the hell of it and it was perfect still!