While I had the guage cluster out I decided to move my boost guage behind the stock cluster.
So bad news, I'll never again be able to tell the time in my Vette. Unless I look at the radio or my cell phone, but at least now I know how much boost I'm making.
That clear stuff is Home Depot plexiglas. Cheap, cut it with a dremel tool. I just used it because I had it laying around. I was originally going to mount it flush with the back of the guage housing, but that threw off the guage alignment because I drilled it up in the air. So I used some nuts to space it. Bolts to the autometer studs and the stock mounting holes in the guage housing.
Also, had to grind down the metal lip around the clock so the guage could sit further up. This is a... 2 1/4" guage I think.
You can just barely see it here. Look for the red needle next to the orange needle.