With an aftermarket boost gauge, I don't need the factory one.  I can now install the stock volt gauge into the hole from my other Supra.
 

I had to remove the ashtray, the dash center panel, the dash under panel, the switch cluster to the left of the steering column, and then the trim around the gauges just to get the gauge cluster out.  I then removed all the screws from the flex print and the white back plastic.  With this off the boost gauge was revealed.  Swapping the gauges is only a matter of two small screws at this point.  I put the gauge cluster back together.  Since the turbo and non-turbo flex print on the back of the gauge cluster is different, I had to jumper the twelve volts over to the volt gauge input.  Then I reinstalled everything into the car.

1- Like I said, the flex prints are not the same.  Comparing my non-turbo one with the turbo, I saw that the 12 volts lead in the print is cut in one spot prior to getting to the gauge.  I just used a length of wire and 2 eyelet crimps to complete the circuit.

None.

This mod is way simple and I like being able to see what my volts are doing (like making the needle dip when the bass hits on my stereo).  Of course having another gauge cluster as well as an entire other car for parts makes this less expensive than it would have been otherwise.