It seems you're passing all the tests for the IVR and the gauge/wiring. Have you got an old intake manifold lying around that still has the sender in it? If so, try the old part. If not, call one of your local Mopar pals and see if they do, and you can borrow it. It really doesn't matter what it's from; the sender was the same from 1963-'89. The factory senders are pretty reliable. Call me a cheap bastard, but I'll usually clean and reuse what was in the car if the gauge worked before the swap. In my Valiant, I changed the sender with a new one (Standard Motor Products #TS17) because the gauge didn't work. It turned out the gauge had failed due to a bad IVR. I found an NOS gauge and it works with the new sender. I'd bet the old one was just fine, though.
One other thing to consider: Did you paint the intake manifold? Paint is a wonderful insulator, and the intake is only grounded through its mounting bolts. Did you cover the sender threads in Teflon tape? You may not be getting a solid ground through the intake manifold. Just for fun, run a jumper wire from battery (-) to the sender body and re-test.