Some of the Expresses had a removable jet in the carb. I believe it was 1979 or some such year that had them. That might help your problems.
Also, it would be a good idea to pour a dabble of oil into your gas tank. The oil injector is only injecting enough for normal running, not an un broken in overbore. Not enough oil means running hot, and that's not good.
I'd also take a peak at your ignition timing just to make sure it's right.