transmission fluid

What is the best transmission fluid to run in the 727 and 904 tranny when changed. The ATF+4 or the Dextron/Mercon and can you mix them. I have a 69 that has only 70,000 miles on it and I had to replace the pan gasket and pan because of a leak. I believe the last time I did this I used ATF+4 but cant remember. Thanks,Bill