73 Dart Alternator doesn't show charge anymore.

What to do about the alternator.
First. Don't trade it in. Knowing the rotor tested at 3 to 4 ohms indicates its probably correct specs. Most of the rebuilt ones are now coming with rotors that measure 1.6 Ohms and draw more current. :(

See what you can find locally as a replacement. At least it will get you going.
You can check the rotor resistance to get a sense of whether they replaced the rotor with a new, high draw - one size for all applications rotor from Wai Wie/Transpo.

Then keep an eye out, ask around for, a used 'square back' alternator. If its in working order, great. If not, what you really want from it is the back casting.

If the rear bearing is OK, everying else can swap over using standard wrenches and sockets. If the bearing is missing or needs replacement, it gets pressed out. New ones are available. There is a special adapter for pressing them, but I'm sure it can be done without if the person is careful. Look at the depth of the old one for reference. Another thing that is done on rebuilds is to throw away the grease deflector. :( If its there, its on the pulley side of the rotor so you won't see it unless you remove the pulley and rotor.