Ok now I did a 350 swap in my v6 87 camaro. I noticed that when ever the tach wire was disconnected from the hei the gauge was within reason, then when you hook up the tack wire (aftermarket tach) the gauge would show way past full. I found a loose connection for the tack wire and fixed it, made the gauge go to alittle below empty(thought it was still broken) I then ran out of gas tinkering last saturday(so it must have been acurate). I went and got 5 gallons of gas and only went to about 1/4 tank? Then started the car and within 30 secs the gauge went way past full again. Any thoughs
The gauge worked perfectly before the swap also.

2.what is the larger purple wire going from the dash towards the rear along the drivers side sill?

 

You need a V8 tach, the V6 one will read high since it's expecting 6 pulses per distributor revolution instead of 8. You can either swap out the whole gauge cluster from a V8 car, or just swap the tach - it's pretty easy to take apart and change.

Not sure about the fuel gauge question though.

 

its a sun pro tach not linked nor wired into the factory tach