How much power are you actually losing out of by staying flat tappet vs going roller? My 351W block is flat tappet, and I plan on getting an FTI custom cam regardless wether or not I stay flat or roller.
What kind of number difference is there powerwise?
I would say somewhere around 5-10 hp. The benefit of the roller lifter is less friction, and better reliability. Roller cams also have fast ramp rates, which cause valve float... You can get more RPM with a flat tappet cam, also more efficient ramp rates, although they are starting to perfect the roller cam and lifter.