One always does the simple things first in determining a problem...never tear down and engine until you've exhausted ALL other potential problems.
The one thing that I think you have not addressed correctly is your timing issue. You need to resolve that before you go whole hog, tear it down and, in my opinion, find nothing wrong.
I'm betting on timing.
Do a proper timing check. If you can't find the timing marks, loosen the distributor a little and turn it one way or another, while it's running at idle.
If it sorts 'rolls' then the timing is too far advanced, turn the dist back a little. If it starts running rough, then it's too far retarded, turn it the other way a bit. You'll find the 'sweet spot' where it will run the smoothest.
When you find that spot, rev it a little. If you hear a rumbling at higher rpm's, then you've still got it advanced just a bit too much, turn the dist back just a smidge.
When you think you've got it right, tighten the distr and run it, see if it over heats.
You'd be surprised what the sound of an engine will tell you when you change the timing a little. If you don't want to try this (you've got nothing to lose at this point), find an old guy in your area, who has been playing with old cars/tractors for a while. I bet he'll be able to get the timing about where it should be, maybe not perfect but acceptable, just by ear.
I've done a lot this way.
This will at least rule out one of the simplest things to trouble shoot. You might find that it makes no difference, but at least you've ruled it out.
Good luck.
