Unless you have the battery disconnected, trying to measure resistance in a circuit such as the alternator is a lost cause. There is no telling what sort of reading the ohmmeter will give. You might try disconnecting one of the battery cables and using the "amps" function of a digital multimeter to measure the parasitic current draw with all the circuits turned off. It should be no more than a few milliamps. Half an amp (500 milliamps) would be a ridiculously high drain. If disconnecting the big alternator charge lead stops the battery discharge, the alternator is bad- - - -no matter how many alternators you have to swap until you find a good one. You could probably resolve the whole problem by retrofitting to a 10-SI internal regulated alternator and tossing that antiquated original equipment unit into the nearest scrap metal bin. GM's first attempt at an AC charging system was a pretty weak one in the 1960's, and those units haven't improved with age. The newest rebuilt external regulator alternator you can buy will be around 50 years old. You can buy a brand new 10-SI at any corner auto parts store.
Jerry
