Gonna throw a cooler on the GMC just because I think the one built into the radiator doesn't do it's job good enough.

How big should I go? Is there such a thing as too big of a tranny cooler?? Should I get the B&M with the built in fan, or is an aux fan not necessary when it's mounted in front of the radiator?

I'd get the biggest tranny cooler that you can fit. I don't know if you'll have room for the tranny cooler with built in fan. I'd get the B&M style and mount it in front of the condensor.

Bypass the in-radiator one or put a valve on it so you can manually bypass it (during the summer) and use the aftermarket one as your primary. You can then redirect the fluid back through the radiator during the winter to pre-warm the fluid.

junkyard tranny cooler - find a 80's deisel BMW. they have an engine oil cooler. 2' wide, 4" tall, 4 cores. works great. ;)

b&m all the way i have two on my dually from the factory well a b&m style:flipoff2: