CAB,
I believe that the 948R is the original generator for your truck. I have an old Army maintenance manual from about 1944 that indicates that the 948 R would put out around 20 amps at 8.4 volts at 2800 rpm when hot. This would be with the field terminal grounded.
The third brush setting is what limits the max current output because it controls the max field strength of the generator. Since 20 amps is the rated max output, it is important that the generator's 3rd brush be set so that this output cannot be exceeded. A good generator shop can do this for you.
Anyway, you should then be able to use a more modern voltage regulator to control the charge rate of the generator because the vibrator that controls the field strength will reduce the current output as the battery voltage comes up, but will not be able to cause the current rating of the generator to be exceeded because the third brush setting limits the max current output.
So if a 20 amp generator meets your power requirements, you should be fine. A bit of rewiring will be needed as you will no longer want the light switch resistors in the circuit. There is no guarantee that this will work as well or as efficiently as a two brush generator and matched voltage regulator, but it should work.
I can email you this army tech manual if you like. It is very cool and has an excellent explanation of how all this works.
I don't imagine that you have two cutouts connected to your generator, so I'm thinking that either one is disconnected, or one of them is something else. If you study it carefully with VOM in hand, you should be able to noodle it out. A picture would be helpful as Jerry suggested. If you don't have a photo hosting account somewhere, I could host the picture for you on my server so you can post it to the board.
Kurt
