I dont know the 3.5 at all, but i would assume a fairly easy way to tell whats going on is to pull either the oil pressure sender, or the line if it's a manual gauge off the block. Crank the motor by hand or crank it with the starter (coil disconnected) just to turn the oil pump a few times. If you get oil out the port, your oil pump is working OK (or barely) and perhaps it's your gauge that's bad. You may want to have a secondary back up test gauge to see if you get the same reading as the dash gauge.

If you get no oil out the port, i'd suspect a bad oil pump

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And yes, this could get a little messy. A 2nd pair of hands with a catch container or rag would be helpful!