temp sending unit/gauge problem

[Justinjob]

Someone replaced my 67 K10's 327 with a 350 before I bought it. Unfortunately they never hooked up the stock temp gauge, or any gauge for that matter. Anyway, I would like to know what my engine temperature is doing, so I wanted to replace the sender and plug in the green wire-simple right? I ordered the original style sender for a truck with gauges. Upon taking the old sender out of the block, I discovered that the new unit's threaded section was larger than the one I removed, way to large to fit in the block. Sooo, without really doing any research and thinking that any gauge style GM temp sender would work, I went down to the parts store and looked in the Wells catalog, I bought a Wells TU66 sender-which applies to most GM and AMC vehicles from 1979-92-it is a single wire hookup for a car with gauges, and it looks identical to the one I pulled out of the block. I put it in, refilled the coolant, fired it up and took it down the road. At first, the gauge did not move at all, making me think that the wiring or gauge was faulty(I was driving long enough for the truck to get to operating temp). Then, all of a sudden, the gauge needle started to rise, it took about 10-20 seconds, but it buried itself to the hot side of the gauge and would not move from there. I'm pretty positive my truck does not have an overheating problem, I've owned it more than a month and it has given me no problems associated with overheating (you can usually hear antifreeze boiling, oil pressure usually drops somewhat, vehicles usually don't run well and stall when overheating) So I'm positive the gauge is lying. The only thing I can think of is that the sender and the gauge are not compatible, maybe the sender is proportionally different? the spring inside is samller and weaker than in the original unit? I even thought that maybe I picked up a sender for a car equipped with dummy lights since the gauge rose so quickly, but I'm positive I didn't. I don't know guys, but I'd like to get it figured out, I don't like guessing what temp my truck is running at. Any ideas????