v12 stumbles above 4500 RPM

Can't argue with your logic. One thing you might try is moving the tach wire from the "-" terminal of one coil to the "-" terminal of the other coil to see if the tach behavior changes. However, it would be best to measure the dwell of each point to see if it's stable/consistent or not -- e.g., if the distributor shaft has a lot of mechanical runout this can cause trouble. Good Hunting!

 

I won't claim to be an expert but I do have some experience with the Pertronix points replacement units and their matching coils. My car has operated from sea level to 11000' and temps from below freezing up to 90 degrees and has never missed a beat. It starts pretty much instantly whether hot or cold and pulls like crazy up to 6500rpm. Also the entire setup is not very expensive.

 

Assuming that the dwell is sufficient (see Steve's post) then this might indicate a problem with one or both of the two BAE201A ignition coils, sounds like a short on the primary or secondary windings. Your tach is triggered by the kickback voltage from the primary coil, so this also indicates a loss of spark energy. Steve's test will show which coil is not working correctly.

Also make sure that your distributor cap is clean and free of dust, dirt or cracks. Broken spark wires or a defective distributor cap (cracks, cross sparking due to metal dust or humidity) will also affect the tach signal.

If the dwell and points are set correctly, and your spark wires and distributor cap are ok, you might want to exchange both coils against a set of new ones. Bosch Red coils (0 221 119 030) are a good replacement and you can install them with the original Marelli coil clamp and resistor.

 

No experience with these motors but my diagnostic tree with any old car is (no matter the problem, unless it is something like the prop-shaft fell out or it has a flat tire)

Check the points (visual inspection)
Check the dwell (timing light)
Check the plug wires (in the dark with a fine mist of water, look for sparks)
Check the points again (pull dizzy and observe everything working to spec by hand)
Check the spark (pull a plug and ground it against something not near sensitive electronics and observe spark)
Check the points again!
Check the cap and rotor (Nice and snug, no moisture, no pitting, no black marks, clean inside)
Check timing at idle
Check timing at full advance
Check timing at idle again
Check timing at full advance again
Check the points again!
Check the distributor again. Does it spin freely and smoothly, without any wobble?

In your case, I'd also disconnect the tachometer and see if you've still got problems. Aftermarket anything is always an easy place to check for problems -- disconnect it and see if the problem persists... Your ears should be good enough to see if the problem remains.

Okay, now check the rest of the stuff like mixture and carb balance and similar stuff.

Some of these rules may be slightly amended if the car has electronic ignition, and if you've got a pure kettering ignition system then you should insert a "check points" in between every step (even if it means you check the points 3 times in a row...)

 

way to much, 350US should cover the conversion, (not including labor)