The first thing that I would do is convert it to electronic ignition and make sure that you are getting adequate spark.  Here's the place that I've used, but there are other sources - Pertronix .  Even if spark isn't your problem, this will definitely make for a nicer running gas tractor.  I was having problems with my Ford 601 fouling out the plugs and after much messing around and rebuilding the carb, somebody mentioned this and it's been fine since.  I replaced the coil at the same time, although you may not have to.

HTH,

Mike