I missed the line where you said you used a thermal gun for checking temps. While you weren't specific about temps at various places, it's hard to say if you have a problem with the radiator. In my experience a partially clogged radiator doesn’t usually cause problems until you run at speed or otherwise under load.
As squeeze said it’s possible you got the wrong sender, or as I said you got a bad one; it happens. I took a quick look at an online catalogue for sending units and for your year they only list one. A sending unit is about 7-15 bucks, so that might be the cheapest approach.
I would hate for you to spend a bunch more money without being certain about the cause. In your case you don't really have a benchmark to compare things against; except that it ran ok before you started. But if you do have a good radiator, as you seem to think, there are no leaks anywhere then that only further my thoughts that it’s a inaccurate gauging issue. I'm curious about your new gauge, perhaps I'm reading too much into this, but I am guessing you are not using a factory style replacement. Some of the gauge sets offered at your FLAPS or other stores aren't that good. Like most things, you only get what you pay for and not all gauges are created equal. Stewart Warner has a very good, long standing reputation, as do some others. Perhaps a post in the electrical forum might yield results of more good gauges.
