Hi Shawn. This has all the markings of being lean in the primary jet, with a tip-in and cruise surge issue. I can't count how many late 70's manual trans vehicles I had to tune with this exact issue.
In Holley's infinite wisdom, on this carb, they put a big nozzle in to cover for a lean primary jet. As long as the accel pump is working correctly, which I think it is...the #31 nozzle should not be the issue.
I know it's timing first, but this has all the same character issues that i've seen before. Assuming the carb has stock specs, i'd jet up from the #64 to at least a #66 primary jet.
I also have my own way of adjusting the A/F mixture screws on Holley's.......Turn one in slowly clockwise until it stumbles and then back it off until it just smoothes. Go to the other side and do the same. Now go back and do both again and when you have them at that point, open them 1/8th a turn and your done.
The beauty of getting it rich enough is, you can probably feed it some more timing and it will run even better.
Just my thoughts.....