Nope, no problem splicing additional wire as need on the transmission temp gauge.
Also, when you install the sensor "T" consider doing it were the transmission outlet line turns upward to the oil cooler. Anywere after it starts upward will do.
Next they are compression fittings, and can leak if they are under tightened or over tightened.
Snug it up just enough to prevent drips or "sweating". You may need to take one or two "runs at it" to get the snugness right.
I recommmend the 'verticle' run because the horizontal section is snug up against the other transmission line, and against the bottom of th bell housing/inspection plate and that is a crappy place to try and get those large open end wrenches in to tighten up the compression fitting. You need to make the final call but keep the issue of accessability and convenience in mind when it comes to install and working space.
The pyro probe should be mounted in the "back" port of the manifold outlet. This is often the hotter port since it vents the rear cylinders which run a bit hotter than the front 3.
Your probe, when installed should protrude into the exhaust stream about 1/4" to 3/8". That is plenty.
Last, when you go to drilling you have a couple options. One is to do it with the turbo on the truck. If that is your choice, then do this:
Center punch about 1 1/4" back from the gasket surface and just slightly left of center. When you drill, coat your bit with thick grease. This will snag alot of shavings as you drill and you can clean it off as you go. When you "break thru" you can use a mechanics magnet to fish for any shavings inside. Then use the same method while tapping..... grease the baby up, tap, clean, grease, tap etc. Again, use the magnet to pull any shavings out.
Don't worry about any tiny bits that "may" have eluded you, they'll blow out when you fire it up.
The other method is removing the turbo so you can actually see inside the manifold outlet. all you need to do is stuff a big rag into the rear port, drill, tap, and pull out the rag. Good to go.
Hope that helps... .
bob.
Hope that helps.