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Temperatures can vary quite a bit. I'll see 450-475 going down the road at 55MPH. After I glued my intercooler boots I saw a dramatic increase in boost and lower EGT as well. I drilled a hole below the clutch master cyl. I'll take a picture later on if you want. I didnt use the clamp pre turbo. Just drilled a hole in the manifold! I think it would be hard to get a drill in there to put a clamp in the up pipe.
I ran my wires through the rubber boot around the steering column. Just poked a small hole and pushed the wires through and it sealed itself around the wires.
Drilling the exhaust manifold is easier than you might think. It's best to drill a very small pilot hole, using plenty of penetrating oil as a lubricant and then drill out the final size for tapping the threads.
IMO the threaded connection is the best because there is NO WAY for any leaks to develope. The hole and clamp method in the up pipe, seems to me, would allow a bit of exhaust pressure out around the probe.