I just got an 84 gl that had a bad dash. The previous owner had the alternator kill two batteries before he figured it out.  It killed the power to the whole thing. I found one short on the back of the dash and fixed that, giving me power to the dash but the oil pressure gauge pegs as soon as you turn the key. Anyone know how to fix this? Ive heard its a not unheard of problem and I have a spare dash or two for parts.

That is correct, the gauges can't be to reliable sometimes.

Could also be caused by the oil pressure sending unit.  When they go bad the result is erroneous readings at the gauge.

I'm thinking its a bad component to the dash but Ill check the sender and wiring. The dash was fried by the alt and thats when the prob started but I'll double check everything.

... the oil pressure gauge pegs as soon as you turn the key...

 

In my case, that issue was caused by a short circuit on the wire that comes from the Oil Pump's sending unit, to the instrument cluster, so I kindly suggest you to start by checking such wire, starting at the sending unit... maybe it is disconnected, and touching bare metal... (Ground)

 

Kind Regards.

I'll second Jeszek on a possible grounding issue. Pegging is usually a symptom of a hard grounding.