While I'm not an expert on these things, I learned a lot from my own trials and tribulations with the ignition on my 71 M code 4V, which is a bit different to the later 73's.
The distributor is just a mechanical gismo and for sure it could be shot, but before going out and buying one of those fancy expensive "brand name" dizzy that everybody thinks are better, I'd do some basic checks. The first thing is the shaft. Is there any movement sideways on it? If there is then for sure the timing could be wandering because the points gaps is changing. If there is no movement sideways, check for rotational movement. If there is then it could be the drive gear is worn also causing timing troubles. If all is good, then perhaps invest in a Pertronix Ignitor I and ditch the points. This module can be run on the reduced voltage, same as the points. It's an easy swap and will give way better performance at this stage. You can upgrade later if you wish to a Pertronix Ignitor II. You will need too buy the matching coil in either case. (the PII needs a full 12V, so other work needs to be done first).
Next you will need to verify the initial timing. I'm not sure what that was spec'd at, but probably 10 deg BTDC is a good start point. You'll need a good timing light, one with a dial to see your degree settings.
If you do decide to replace the dizzy and if I had to do it again, I'd buy the Ford DuraSpark which is a bit later version of the Motorcraft you have, but electronic. You can buy these already set up for your engine specs, but I don't remember the name of the company that does them. You can also buy a reman from Cardone or their new Chinese version.
This can all be a bit daunting, but I get the feeling you're a very capable person and can figure it out. Ask lot's of questions if needed.
Good luck.
