I'd mount it wherever it's convenient, especially if you aren't using it to really monitor engine conditions. I drilled and tapped the cast O2 housing, and mine seems to be holding up fine.
Using EGT as a tuning guide is outdated, IMO. Most people used EGTs when they were a cheaper and easier alternative to WBO2s back in the day. Nowadays, for what it costs for an EGT gauge/sensor, you'd be better off running a WB in each runner if you're really concerned about each cylinder is doing.
A 12.0 AFR is a 12.0 AFR on every engine. Pick the ideal AFR for your engine and you're off and running.
A 1250* EGT is a 1250* EGT on every engine, but there's no ideal EGT.
Optimal EGT will be different for every single application, and it typically peaks around stoich. That makes monitoring for peak temps pretty useless for tuning AFR because nobody wanting to make power is at stoich. Since temp ends up dropping off on both sides of stoich (until knock occurs), you'd need to figure out if you are on the rich or lean side. That's why I've heard tuning in the old days referred to as an art...Since not every kid who has $545 bucks and reads the ECMlink Wiki could do it. EGTs provide a good overall reference when coupled with a WB, and are also a very good way to verify cylinders are performing the same across the board when stuck in each runner.
A WBO2 will give you true AFR. A EGT can give you relative AFR. That should make it pretty apparent why few people are relying heavily on EGTs for tuning these days. Our generation is spoiled with the superiority of cheap and awesome technology like the WBO2. I basically use my EGT to fill the "other" hole in my gauge pod just like Wop /ubbthreads/images/graemlins/rofl.gif