Will the fuel gauge on this work with the sender on the M1A1 with in-tank fuel pump (9664SKNET), or does it need an adapter for the sending unit included with that tank?

Yes, you should have no issue. We stock the Classic Instruments gauges with the stock Ford range, 10 -75 ohms. The M1A1 sending unit is the same as stock 10-75 ohms. If you source the gauge elsewhere, make sure it is set for that range.