Oooo! I can answer this considering I just bought 52mm Greddy boost and egt gauges (electronic) and a dual instrument cluster bezel. I don't really understand what you mean by wiring electronic gauges to the stock harness but here is how I wired. I took the red wire and put it in fuse slot 8 I think which is the brake light fuse which should always have power. I grounded the black wire. I put the orange wire in the radio fuse, I forget which fuse slot it was but you can check your owners manual. And I put the white one in the fuse for illumination brightness control. Everything works great that way. Gauges goes all the ways to 760mmHG or whatever when car is off and the backlight goes on when I turn my lights on.
The T-fitting should go right off the stock bypass valve. T it a few inches off that and make sure you have that brass restrictor securely in the T-fitting side that goes to the boost gauge. I mounted the sensor on the empty stud on the battery.
As for the EGT sensor, I think 3rd cylinder is the passanger side one, closest to back of car. So the closest one to passanger seat basically.
Hope I helped.