newish to me built 340 valiant feels sluggish. Timing?

Nobody is saying don't run vac adv, just don't time the car with it connected.

Closed chamber edes? The engine will probably like about 34-36 total, maybe less, without vac adv.. You could set the initial at 14-16. It may even be able to start at 19-20 initial (this would require changing the rotary switches) This is where I don't like the e-curve, you have to make 5* changes in the curve, no in between selections available.

Set initial timing, simple. Reset idle speed and mixture screws if it speeds up when you do this. Hook up vac adv line to dist can.

The only other change, the setting on rotary switch #1 from 4 to 0. That will slow down the vacuum advance rate.

Verify the timing without vac adv, if currently incorrect, it may drastically change the performance of your car.