Bought some Livorsi mega rim gauges to replace the factory ones in my 89 311. Changed them all over from old to new Livorsi using all the factory wiring. The Livorsi have the push in plug on the back. Everything works but the oil pressure gauges go straight to 80psi and the fuel gauge goes to half tank once you turn the keys on. I know the fuel tank is bone dry.
Anyone know why?

You are going to need to change the senders for both the oil pressure and fuel gauges. VDO senders for those two gauges are more or less backwards from Livorsi. The oil pressure is an easy fix, as I recall you will need to do some cutting by the back seat to get the fuel sender out.

Yeah, to get the fuel sender out you will need to remove the lowest fixed cushion on the back seat and drill an access with a hole saw. When your done, putting the cushion back will cover the hole you made.

So do I have to use the Livorsi fuel sender or will any fuel sender with the correct ohm rating work?

What about the oil pressure sender? Does it have to be a Livorsi or will a factory gm sender work?

I had a VDO oil sender on my Pantera with Livorsi gauge when I bought it. It did work backwards as stated.
I don't know if a GM sender will work. I replaced it with a Livorsi one.