engine cutting out down track after in 3rd gear!

Seems like one fundamental question has not been answered: fuel pressure. This carb uses 5-7 psi, not any higher, but it has to be steady. Pressure going low or high during a sustained fule use for any reason will cause your symptoms, and either could/wouldshow up at longer times at sustained fuel use, like in 3rd gear. A timing advance problem would show up in all gears.

I can't find anything in the OP's post where this has been measured for sure, and thus confirmed or denied as the issue. You're chasing your tail 'til you answer this, and wasting time to change carbs with this unanswered. The orignal post says that the original regulator is 'only 6 psi', making me think that the OP thinks he need more; not true! Make sure you are regulating at around 6 psi, and then put a remote fuel pressure guage at the carb inlet and test it during a run.

And OBTW, there is a fuel filter in most inlets in Holleys; check those for being plugged.