I believe it'* 1/8-NPT (1/8-27 national pipe thread) but I could be mistaken. I don't remember if it'* a taper, but the sender from Cyberdyne was a perfect thread match.

Edit, cyberdyne lists it as 1/8 NPT. That'* correct.

http://www.cyberdynegauges.net/accessoriesproduct.php

One of the other reasons I prefer cyberdyne is their option for high-low memory recall, and user-adjustable warning points.

Part # A221E061Y-Black Bezel w/memory
RED Transmission Temperature
Requires 2 1/16” hole for mounting with back clamp provided. Range is 70-260F. Features ‘Hi” and “Lo” scrolling warning and user adjustable “flashing” warning. Available with high and low memory recall feature. Requires Cyberdyne temperature sender shipped with gauge. Order from part number list below.



$59.99 with memory and the sender at Jeg'*.
http://www.jegs.com/i/Cyberdyne/280/A221E061Y/10002/-1

I've been towing travel trailers for a number of years now and have learned some lessons the hard way....
Keep in mind that if all you have is the stock transmission cooler, (note the transmission coolant lines going into and out of your radiator), then your trans will never run cooler than your engine and will often run quite a bit hotter when going up long, steep grades or towing. Transmissions in the other GM cars, vans and trucks that I've had, can run safely up to 240 degrees, however I've been told by transmission experts that you should never run transmissions over 250 degrees. I had a trans temp gauge on my 92 Chevy van (after blowing up two trannies towing a trailer with it). I discovered that if you can manually lock your TCC (which was easy on the OBD I GMs) your transmission temperature would drop quickly back down to engine temperature. The excess heat in your trans is generated mostly by friction in the torque convertor as the engine side and the trans side spin at different speeds when not locked up by the TCC. You want this "slip" most of the time, it gives you power multiplication, like slipping your manual clutch. But it makes heat too, just like slipping a clutch. Another source of heat is when the trannie shifts back and forth between gears a lot--happens on long hills as gears are constantly releasing and engaging, which makes for slip and thus heat. So if you're climbing a steep hill you're probably better off keeping the trans in the lowest gear it wants to go into (i.e., it keeps shifting into 3rd in order to make it up the hill) instead of letting it shift back and forth.
Anyway, bottom line(*) 1) Your stock trans and cooler are designed to run cool enough if in good working condition, even on hot days and up long grades (unless you're towing). If they don't, which you won't know without a trans temp gauge, then you might have internal problems that should be corrected. 2) Knowing your trans temp is only half the battle. You also need to be able to keep it down if it'* too high. Aftermarket trans coolers, installed (in MY opinion) on the return line from the stock cooler or, if big enough, instead of the stock cooler are one answer. If that doesn't work, or at least not well enough, can you put in a switch to manually lock up the TCC? 3) Not only that, but is your cooling system in good working order? As noted before, your trans is tied into your engine cooling system. 4) Is your engine running right? An engine that is running poorly will generate more heat than a well-running engine.