Would this be with a vacuum secondary type carb? Just throwing it out there.....
I might be inclined to put a vacuum gauge on the intake and see what's going on at the nose-over.Or actually, what's going on just before the nose-over.
'course you could just cut the ignition, when it pukes, pull over, and check the bowls.
'Course at 5500rpm this is 92 revolutions per second. A good system with a mechanical pump would at least half fill one bowl, having pumped 120gph(freeflow) x 32 ozpergallon/60minutesperhour/60secondsperminute = 1.07 ounces/second. Multiply by 1/2 cuz the cam pumps once per 2 engine revolutions =.533 oz per second. Then say an estimated 3.5 second shut down time; 3.5 x .533 =1.86 oz.I think that's about 50cc or nearly one holley-type bowlfull.
So if the bowls are empty, or very low,it would be a bad pump or system. But if the bowls are full, it could be either or.
If some amount of the fuel in the rear bowl was stacked up against the backwall of the bowl, exposing the secondary jets, you wouldn't know this, cuz you have already backed off the pedal, and are trying to recover on the primaries.So recovery time on the primaries might be 2 to 3 seconds to where the engine begins to respond again, and another 1 or 2 seconds to to fill the secondary bowl,and then it will take full throttle again.
I can agree with others as to the Hi-po pump, and a 3/8 line all the way into the tank, as being adequate for my 367 cuber, which goes 93 in the 1/8. Doing the math backwards this calculates to about 420 hp.
As to tank venting; consider that at .5 lb per hp per hour(you do the math, using average output for the run) this equates to pulling just a few ounces from the tank, per run. If you start with a half a tank of gas this will not present a problem for the Hi-po pump, if unvented. It will probably have enough power to collapse the tank.