One check you could make, and its easy, remove cover from regulator, assuming you still have the standard regulator, run the the engine, check and see if the points close on the cut out regulator , the one nearest to centre of truck, they should close when you take the up revs up, if they dont, fault could be with generator/wiring, if they close, fault could be with the regulator.
WORD OF WARNING. Do not be tempted to close cut out points manually, with engine not running, this will make a circuit between battery and generator, and will try and turn the generator, like a motor. I remember this happening in Holland many years ago, a lot of smoke in the engine compartment, the truck belonged to a regular member on this forum, who now resides in Suffolk.
Hope this helps.