I got Diprocol gauges from GOS racing also. I don't remeber the instuctions being lousy. Maybe you didn't get them all? Anyway I would put the probe in the divers side exhaust manifold. That's my opinion. Here are the pics of my probe.
As far the tap/bit size goes...I just got mine out and looked at them. It was
1/8" NPT for mine, not 1/4" NPT. The drill bit size I finished with was 21/64.
You're right, you got one shot.
Do you have the taps already?
If not bring your probe to the store with you and compare the size of the probe with the sizes of the taps. 1/4 NPT is huge. You'll know if its 1/8" or 1/4" when you see them next to each other. Good Luck and post back with any other ?'s.
Here, this may help too.
Pyro probe install
I had about three or four links for gauge installs before I did mine(i'm anal about that stuff), i'll look for more if this info doen't help you...
later