Don't fret the 'fit.' Buy a quality brand unit and just use barbed hose fittings to splice it into the factory lines. I put mine on in 1996 and never looked back. Mine sticks in front of the factory radiator/condensor held in place with nylon thru ties. [I am not in favor of the horizontal installs on frame crossmembers - you want ram air flowing head on to and thru the cooler fins.]

Just cut the rubber hose that feeds the ATF from trans to radiator, put in 3/8" hose-to-hose barbs and add tubing to the cooler and back. I added an in-line filter kit using full size oil filter; adds ~1 qt extra ATF system capacity as a bonus to external pan filtration.

I did not install a gauge but with the same method you could splice one in. Where is the factory ATF idiot light temp sender? Can you swap that out for an aftermarket sensor/gauge?