Help, with a friends 67 Cuda 340 issue

Yes, the condenser was changed also.

The power valve is correct, no change from before the replacement.

No vacume leaks, close the choke with it running and it will die.

No slack in the chain, we turned the engine with a socket and it is tight,

no clattering or anything.

Forgot to mention that this is a pretty fresh motor.

It sat for 20 yrs, then was taken apart, rebuilt 60 over, and put together again last yr.

Its really a evil car when its running right.

Leak down test time.
When the rings/valves 'mainly the rings' go south...the power go's flat.

how does the mark look with a time light, does it move around a lot?

you didnt mention the coil, so you could start on the cheap and test/change that.
Fuel pressure should also be tested.