So if I put battery wires to battery and ground the sending unit wires they should read FULL hot?
Yes, but that's not really a good idea. Inside the gauges is a little heater, designed to run on 10 volts or less. Hitting it with 12.6 volts from the battery may not sound like much, but it produces over half again as much heat and can damage the gauge.
If you must do the test, just brush the wires together long enough to see the needle head for the stop.
As noted, the original MGA gauge used a "balanced" movement, not the "hot wire" found on TR4 and later TRs. The balanced movement actually reads backwards, grounding the sender wire should cause it to read empty (so the problem might be a bad connection between the gauge & sender, or a bad ground on the sender). But the car might have been modified with a later gauge. I'm not sure offhand if all MGB used the "hot wire" movement, but most of them did.