Holley 750 "stumbles" on WOT

If it ran fine with the tune up and just started doing it. I'd look at the accel pump diaphram. Fuels are hell on them now.

Push down on the arm and it should move nice and smooth. Providing a nice shot of fuel. If it feels stiff, the diaphram may have gotten brittle. Then do it using the throttle linkage and make sure it porvides a good pump shot.

Double check your ignition timing, make sure things haven't moved.