Check if its got points in the distributor and a condenser bolted to the side of it, it should have with this serial number if it has make sure your tech changed both as quite a few just change the points, a fault with either will cause this problem. Get the ignition timing checked as well as a rule of thumb slacken off the clamp bolt start up the truck and rotate the distributor clockwise until the tick over note changes then anticlockwise until the same happens then set the distributor in the centre of the 2 and give it another try if it improves get somebody with a timing light to set it properly. If all else fails the carb/mixer it should have is a Ca50/55 these are prone to body wear (when the XL range came out we changed lots of the things) so youll be looking at a new one