Have you checked the vacuum advance to make sure it's working? Maybe bad wires on the pickup coil?

75 should have a cat.

The stock single barrel carbs are well known for issues, usually a worn out throttle shaft, and good rebuilts are almost impossible to find.

The stumble on acceleration could be accelerator pump, clogged fuel filter or broken fuel pump. To many variables to diagnose without being there.

If you are looking for a rebuild kit, check out these guys. https://www.carburetor-parts.com/

I'm betting it's a timing issue. Try advancing the timing. Even if you don't believe me, give it a try. It's cheap and easy to try. I've been down this road before.

I'm not looking for a rebuild kit the carb has been rebuilt. It has a new fuel filter and I have a pump to put on but haven't done so yet because I figured it was unlikely.

The mid 70s were the worst carb issue time that ever existed. Even new carb quality was all over the place, emissions were stepping up every year, and the gas companies were messing with the gas all the time. The chokes had electric circuits added to pull the choke off faster to meet the emission standards, and the car companies were screwing with carbs, ignition timing, and an array of EPA demanded add on stuff that also changed every year, some of which didn't work when new.

That stumble and hesitation was a common problem. To address it from a full cold motor to a full operating temp was always a struggle of compromise, you had to pick the point where the customer was the most happy. Usually a stumble or hesitation on a fully warmed up motor could be adjusted out adjusting when and how much the accelerator pump would discharge the extra fuel kick when the gas pedal was stepped into, and that adjustment was not based on how you drive, but how the customer drives. A very light footed owner is very difficult to adjust an accelerator pump for. If I was adjusting a stumble for a very light footed customer, my boss had to do the test driving while I rode along to feel and hear it.

Without knowing the operating temp of the motor when the problem is occurring, fixing it online isn't going to work out well. I need to know if the problem is when the motor is stone cold, at operating temp, or somewhere in between, or is present through the entire operation.

I was the carb adjusting guy at the largest service garage in our town from about 74 through 79. Any stumbles or hesitations came to me, even from other garages. Proper adjustment was not a one time and done process, and in that era, it was often also a fall and a spring adjustment as well. It is an act of several fine adjustments no book is going to tell you.

For the record, just because the carb was "rebuilt", that does not mean it was rebuilt correctly. Throwing new gaskets into a carb does not rebuild it.

There was a reason the car companies went to electronic fuel injection.

With the engine at operating temp.
From a dead stop, normal acceleration, If the car momentarily responds, stumbles, and picks up again, It could be the off idle transition is not responding as it should.

If it stumbles immediately and recovers it usually indicates an issue with the accelerator pump circuit. Could be misadjusted, incorrect nozzle size or several other things.

Your surging could be due to a lean condition caused by several things, vacuum leak or excessive timing.
Going to need to find someone who understands the older vehicles or dive in yourself. As mentioned most rebuilt carbs are sketchy at best. the onlycompay I've had any luck with is Auto Line out of canada. Rock Auto sells their carbs. they will also do yours if not available
keep us posted

I'm betting it's a timing issue. Try advancing the timing. Even if you don't believe me, give it a try. It's cheap and easy to try. I've been down this road before.

The mid 70s were the worst carb issue time that ever existed. Even new carb quality was all over the place, emissions were stepping up every year, and the gas companies were messing with the gas all the time. The chokes had electric circuits added to pull the choke off faster to meet the emission standards, and the car companies were screwing with carbs, ignition timing, and an array of EPA demanded add on stuff that also changed every year, some of which didn't work when new.

That stumble and hesitation was a common problem. To address it from a full cold motor to a full operating temp was always a struggle of compromise, you had to pick the point where the customer was the most happy. Usually a stumble or hesitation on a fully warmed up motor could be adjusted out adjusting when and how much the accelerator pump would discharge the extra fuel kick when the gas pedal was stepped into, and that adjustment was not based on how you drive, but how the customer drives. A very light footed owner is very difficult to adjust an accelerator pump for. If I was adjusting a stumble for a very light footed customer, my boss had to do the test driving while I rode along to feel and hear it.

Without knowing the operating temp of the motor when the problem is occurring, fixing it online isn't going to work out well. I need to know if the problem is when the motor is stone cold, at operating temp, or somewhere in between, or is present through the entire operation.

I was the carb adjusting guy at the largest service garage in our town from about 74 through 79. Any stumbles or hesitations came to me, even from other garages. Proper adjustment was not a one time and done process, and in that era, it was often also a fall and a spring adjustment as well. It is an act of several fine adjustments no book is going to tell you.

For the record, just because the carb was "rebuilt", that does not mean it was rebuilt correctly. Throwing new gaskets into a carb does not rebuild it.

There was a reason the car companies went to electronic fuel injection.

So 70 Duster, are you saying that 4 degrees after TDC isn't enough? Haha. I just put a timing light on it and that's where it's at. What should I advance it to? I haven't checked anything else yet.

the initial timing is not what will cause the surging but rather the total or timing at 60 MPH. This is where the mechanical and vacuum come into the picture. the combination of initial, + mechanical + vacuum can create too much advance that results in the surging.
DO NOT assume the timing mark is correct without verifying it indicates TDC when the #1 cylinder is at TDC. Slipping outer balancer rings, mismatched components can lead to erroneous timing.
Once you verify the ignition/ timing you ca then move on to the carb

1975 emission systems had a factory work around where the egr was disabled and the vac adv hooked up to unported vauum etc. I owned such a beast and had the factory docs.
Not anymore so in the meantime disconnect the egr, put the vac adv on ported

- you do not have the car set up properly with the timing so please pay attention.

Give the timing as much advance as it will tolerate without detonation

I literally started messing with the timing yesterday at about 7:30 PM, it's a work in progress. I am working on a car that isn't mine. I don't know who has done what to it aside from what I have done. I am reporting what I find. I chose 6 degrees BTDC based on information gathered from a thread on the slant6 forums website. Some of the posts of the users there are gone so I got limited info for a baseline.

Also any idea which spot is ported vacuum or how I can figure it out?

The ported outlet should have no vacuum at idle and should increase as the throttle is slowly opened.

Did you happen to verify that the timing is returning to its initial setting at idle and not hanging up or "floating" down?

Also verify Total timing.

That's good, disconnect + plug the port. See if the 60 mph surging goes away