Look at it this way, you're figuring out what it isn't...and you've found a lot of things that weren't right in the first place, so they all might have contributed at different times. If the gasket is good, and I'm no expert on this, but I assume if the gasket was in wrong or had another problem, wouldn't the car pretty consistently overheat? Seems like sometimes it's fine and others it's not. I'm not a mechanic and I don't want to give you bad advice on that, just seems like if it was bad or installed wrong it would always be overheating, not just sometimes. Of course, it started acting like this after you replaced the gaskets, which is kind of telling, but also there are a lot of other things that might have changed when you did that job. I hate to have you pull the top end off the engine for nothing. Also find it noteworthy that it overheats when going fast as opposed to in traffic. 75 probably doesn't run much over 2,000 RPMs, it's not like you're wide open there. Not sure what, if anything, that means, it's just strange. But it also might point to something other than the cooling system, for instance is there any way the timing can cause that only at higher speeds? Not sure there, someone with more knowledge than me will have to help you with that.

Oh, and to answer your question about using a tee for the temp sender, i'd say no. If you have a mechanical gauge it uses a different plug. Just unplug the wire from the existing sender and tie it up to keep it off the exhaust, and hold on to the sender. Screw in the other one, and you're done. If you get to a point where you don't need the mechanical gauge anymore, just take it out, and put in the old sender and plug it back in. I didn't miss my old gauge once I had the other one in.