It sounds pretty specific to the spark running the motor. You need to check then recheck the wiring going to and from the MSD box and distributor/coil. I don't think the timing control is something you have to worry about. The tach wigging out is a sure sign that the tach is not recieveing the adequate voltage to read the pulses. A friend has a 67 Conv. that was doing the same thing and we found that the positive power supply to the tach was being shared with about 4 other things on the fuse block. Also, it turned out the alternator was going out on it too. We replaced the alt. and the car ran okay but the tach was still wigging out and when we isolated the tach with it's own power supply, it was fine.
So my other question is this- Is this issue completely intermitent or does it run funky all the time. You said it's getting worse.....as in more frequent or now the motor is running even worse all the time?
I think the engine is getting poor spark an now some of your plugs are starting to foul...Pull them out and look at them...what do they look like? If you have a camera....share.