Nick's still plugging away.
If you want, let him know in his comments, I'm sure he would be grateful.Nick's problem with the loss of power is most likely caused by the new '72-'73 340 distributor which has an advance curve that is actually toxic for the old Hemi. An early '70s smog 340 (or any other LA engine) distributor advance curve is not remotely similar to the curve in the original 1954 241 Hemi distributor and will actually damage the Hemi.
Nick looked at the plugs and thought it was lean, but already saw that richer A/F made less power. The original jetting was right ....... whowoodathunkit ......... the Mopar engineers knew what to do in 1954.
The plug looked lean because it was hot from way too much advance. I think (look it up later) the 241 Hemi curve is only 20 crank degrees and the smog early '70s distributor might have 40 crank degrees. The early Hemi only needs 26 total, so, if the timing is set so the engine will idle acceptably, with the new distributor it will have way ..... way ...... way too much total.
The compression ratio in the 241 is low enough it might not knock but it will be down on power and overheat from too much timing.
That rusty looking original distributor has the right advance setup for the engine and could easily be refurbished to look and work like new, all it needs is some time in an ultrasonic cleaner with some Zep Rust Eraser, maybe new bushings, some paint, and some time in the distributor machine. I've fixed them up that looked worse than that.