I read this same post for tips on drilling before i did mine.
I used a right angle drill, Small bit first for pilot hole did not go all the way through with the first bit. Second bit was a little larger to get more metal material out of the hole before I went through. 3rd bit was the right size for the tap, I did the with the engine running, exhaust brake on , tailpipe taped up half of it to build back pressure, grease on drill bit. WEAR SAFETY GLASSES!
Worked like a champ as the manifold metal is kinda soft.
Just go slow and it will work out. I know what you're feeling as I was a little aprehensive of doing it myself. Once it was done I did my buddies also!

 

i paid my dealer one hour labor to install the egt probe, any issues and they have to deal with it, $100 is not worth my time even if nothing goes wrong. as far as the egt's go mine runs around 820 down the highway @ around 65mph, when it goes into regen the temps get up to 1200 or so, if you idle alot during the day like i do then once in a while your nox regen will kick in, the egts will surge to 1340 on the highway for about 4 minutes and then return to normal. hope this helps

 

installed my westech gauge today boost/pyro. here is the location i picked for pyro and the boost reading. overall it went well just took a long time cause i was careful.

tried it out on the way home going up hill 6th gear not a real big hill it was running 900 to 1000. does that seem right.

 

temps sound about right but it looks like you are too far off center with that hole, you will be picking up 3 cilinders not all 6

 

from what I understand you dont want the egt on the new engines centered likie the old ones. I think the front cylinders run a tick hotter right???

Chris