Forgot to update this. Problem is solved it seems.

Ignition coil was the initial problem. I also replaced the wires with NGKs.

I suspect an issue with the alternator as I smell a burning smell from it but the battery is charging fine and the car will run on just the alternator.

But problems solved. Thanks for your advice and patience guys.

 

Good to know, especially considering "The issue started with the f23 manifold swap." Your troubleshooting persistence paid off. Internal or external coil? In either case, it must be that the coil was shorting when the engine got hot. A common failure 40 years ago but not too often seen in modern cars.

 

I think internal. Inside the distributor cap.

That fix gave me a consistent and strong throttle. Now to move on to other small issues. And there are a few. This car just takes a beating.