Has anybody installed a tranmission cooler and would it be really necesary if a
3000 stall converter is used?
And what about a gauge that shows the transmission temperature?

 

A cooler definitely can't hurt at all. Don't know if it is necessary but will definitely help. I would probably do an upgraded radiator also. Not sure about the temp sensor though.

 

What this guy said. You can either plumb it in ahead of the radiator to take a load off the engine cooling system, bypass the liquid to liquid cooler completely or put it on the return side of the radiator cooler to increase cooling capacity.

If you decide to plumb in a trans temp gauge, the gauge should have instructions where to do so, if not I'd put the gauge on the feed side with an adaptor so that your getting the hot reading.

edit---> with the adapter (err... manifold in gauge guy speak) as close to the tranmission as is possible so that the fluid line doesn't have enough time to radiate heat and give a false reading.