Update on 87 GL rebuild....

It's been awhile so here's the history:
87 GL approx. 200k.  New rings, fuel system, head gaskets, assorted
hoses and gaskets. Put engine back in and started having an oil pressure
problem...Light and buzzer go on after about 20 minutes idle or about 2
miles driving.
Got both new oil sensors new VW oem filter, 20w50 oil... still had same
problem. Decided to put on a gauge...

Got the new oil pressure gauge and the double sender from
Bus Boys Friday. Had to have them overnight the hose connection, due to
them not shipping it like I had asked...
Got it all put on, started it and the pointer immediately raises all the
way to the top of the gauge...as far as it will go.

I think all the wiring is connected right, so here is what I am
thinking:
1. Couldn't get 16 gauge wire, so I used 14...Can that cause problems?
2. It came with a loop retainer, so I mounted it with one of the
bolts that connects the heat sheild, to the engine block...may not be
tight enough to ground? Would that cause this?
3. I actually do have tremendous pressure in my engine...what's causing
it?

As always I appreciate any replies to this problem...Ya'll have helped
me a lot so far...
This is my only vehicle and I really need to get it going...