After pulling, rebuilding, and replacing the engine out of a friend '75 , the amp meter is not working.  I'm sure the alternator is working because it gets hot after running a while.

The alternator getting hot is not really a confirmation that it is actually working properly. I would suggest using a Volt meter to verify charging voltage. Might just be a loose/corroded connection that got disturbed in the process. Check the voltage on the large red wire at the alternator, then check voltage at the battery terminals(engine running, of course).  Battery voltage should be within a volt or two of charging voltage at the alternator. Make sure the connections at the starter is nice and snug, as well as on the alternator, and the battery. I would also suggest making sure the ground cables are tight and clean.