Greg-
I went with 1/8 NPT on mine. The reason was because Murphy's Law follows me wherever I go. I figured that there was the very, very small chance that I could mess up the 1/8 install then I could just go to the next size up and use an adapter bushing. If I started with 1/4 and messed up, then the new hole would be that much bigger (and noticeable). Turned out the install was very straight-forward.
I installed the white faced Westach combo gauge on a SRT pillar. At night the face is still white, but it didn't bother me since it was off to the side away from the main dash gauges.
However, this week I just installed a Dipricol fuel pressure gauge on a steering pod, and it now makes the Westach stick out like a sore thumb, but only at night because the Dipricol changes from day-to-night pretty close to exactly like the dash.
If I would have bought the black Westach, then it would have now stuck out like a sore thumb (probably to me only) in the daytime only.
Maybe there's a local TDR club near you where you can meet up and get help?
Hope this helps!