1972 Demon 340 engine mods

OK so here is an update.

Since I had contact with this car I noticed erratic idle. Two weeks before heads swap I noticed that spark plugs looked coal mine black. I checked timing and it had 15 degrees advance.
The owner told me that he has an issue with driving longer on highways. He said that after some time car wont go over 30mph, looses power and everything goes back to normal when it cools down.

We swapped the heads. I did not touch the distributor in the process, what I noticed was that nr1 cylinder plug was directed towards the driver, as if distributor was 180 degrees out. Also spark plug wires seemed stretched. I started the engine after heads swap and it started right up. Idled as it used to(erratic). I took the car for a ride and only after 10-15 miles spark plugs looked just like the ones he used for a few months. All black. I heard explosions in the exhaust at crusing speed also.
I set the engine to 1st tdc, turned the distributor 180 over and reconnected the wires. I also had to clean the cable terminals because half of them were badly corroded. It started right up, so I went out for a ride, nothing was changed, still idled badly, still felt underpowered. Basically heads swap gave zero power change.
After this I decided to play with timing advance. This made me think as I was able to advance it to 30 degrees at idle and engine speed was getting higher even past this value. Also 30 degrees made it react to gas better(?!)... It started revving to 6000+. Before that 4000 rpm felt like a struggle. I am suspecting some electrical issue, weak spark due to bad coil or something. This car acts really strange.
I will also double check my distributor installation. I verified markings on the balancer are correct with piston stop.

Any guesses? Suggestions?