Sounds like you have an electrical problem. Either a short or a bad ground. Did you forget to plug something in or ground something out that shouldn't have been? Also as you were removing the engine/transmission did you happen to pinch any of the wiring? If you didn't make any changes to any of these components (starter/gauges) then they shouldn't be suddenly having trouble.

Your wideband can effect your ECU, that is assuming it is replacing your existing front O2 sensor, then it is sending AFR to the ECU as well as to the gauge. Technically the gauge does not effect your ECU, it is only there to inform you of what the O2 sensor is seeing. The sensor itself is what is telling your ECU what is going on. Alternatively if whoever installed it put it in as a tertiary (which is more typical) then it likely isn't feeding the ECU at all and only there to tell you what your fuel mix looks like for tuning purposes. It really depends on how they installed it. You could trace out the wires from the EUGO sensor and see if it interfaces with your ECU or just goes into the back of the gauge. Here are some interesting links on either setup.
http://www.dsmtuners.com/threads/ho...-2g-turbo-in-place-of-stock-o2-sensor.432222/
http://www.dsmtuners.com/threads/how-to-install-a-aem-wideband.243242/

As for the gas: Gas is only good for about 30 days before it begins to break down. It will still work but you shouldn't run a bunch of boost on old gas. Dilute it out with fresh gas. If your tank was pretty full, throw some octane booster in there just to be safe and take it easy until you cycle that gas out of there. It's easy to get detonation in a Turbo engine.

Very helpful man. Nothing should have changed electrically but something must have to have caused this mess. I'm going to go through the wiring more thoroughly monday and see if I overlooked something.