6.0 cooler should be fine by itself but yes on the temp gage.
Try doing a lot of low speed heavy towing, especially backing up. Then see how hot the trans is.I know quite a few people who bypasses the radiator section of the cooling system.
They have zero issues with temp increases. They run a 6.0 cooler.
I don't know for certain. The specs are so far apart for MERCON and MERCON LV that I don't see how one fluid could meet both specs.
I do know that MERCON LV will destroy a 4R100. You should have used a MERCON V fluid. Will the fluid you used destroy it? I don't know. You could either run an experiment and see if it dies or not, or flush it out with MERCON V.
Pour 10 quarts in through the dipstick tube. Run the engine in park for one minute and then shut the engine off. Add five more quarts and start it again. Run the gearshift through each position and fill the trans to the middle of the cross hatch area.Mark Kovalsky any pointers for how to fill the new tranny with fluid?