engine cutting out down track after in 3rd gear!

The timeslip is close/right in line with most incrementals.

What is the initial timing. That's one of the first places to look for why it's slow down low. Total timing is one piece, what it has at idle is REALLY important. It would likely need at least 20* timing at idle.

Regulator have small inlet? smaller than 3/8" pipe?

It's at least 10mph slow from what I'd guess it should run, but it's soggy from the get go as well. There's a bunch missing everywhere. Get to tuning it up.

Fuel delivery, ignition and valve springs would be the three things I'd be looking at for potential issues.