I have an 86 iroc z tpi. I pulled the 305 out and dropped in a carbed 350. My fuel level gauge, alternator gauge and speedometer all work, but the tach, oil pressure and water temp gauges do not. The previous owner cut the EFI harness so th is wires are gone forever. I'm using aftermarket mechanical gauges right now.

My question is, is it possible for me to use the factory gauges by rewiring them straight to the motor or do they only function properly when run through the ecm?

 

The ECM is not involved with those.

Find the wires and hook em back up, they'll work fine.

There should be a special level in Hell reserved for people who cut wires. They should spend their eternity fixing things that have been hacked up that way by others. Maybe there should even be a few more torments added though I can't think of many that are worse.