Only keeps about 11.7-11.9v while running at idle.
Is it a problem if I get a new dual field alternator and ground one field like I have right now? If I get a single field I’ll have to redo some bracketry.
That does seem a bit low. While our alternators do have a problem with low speed output, with no electrical load other than the coil, it should be higher than 11.9vdc. Did you check voltage at a higher rpm, like 1800 to 2000? Going to the later regulator with the alternator is a better plan.