1975 Volkswagen Van Missing

There could be a few issues, first of all what kind of plugs that you install? I like to run the factory specifications, because those platinum plugs are non compatible, specially on older German cars, the next issue is the adjustment of the valves, I don't like putting them to factory specifications, because when it gets hot it doesn't run right, the way I like to adjust them is, down to no play and that's it, factory specs is no play one turn, I don't advise that, another issue is, have you checked your timing? If the timing is too advanced it will cause it to run hot, and since you recently had the heads off, check and make sure your cooling ducks are all hooked up, and make sure the heater doors are open, and that your gasket runs all the way around the engine compartment.