My trans pan was ridiculous to seal up, and it took forever.
Didn't work with RTV/gasket at all, didn't work with rubber gasket alone. Multiple bolt holes had stripped out threads, causing the bolts to just spin. After tapping three new sets of threads and getting new, larger bolts to fit in them, I used the rubber gasket and spread dielectric grease on both sides. I then put blue threadlocker on each of the 14 bolts, tightened to spec with my in/lb torque wrench, and played the waiting game for 24 hours while everything cured (before filling and/or running). Shift kit installed, and happily NO LEAKS!
I would also highly suggest installing a drain plug. Only about $3 at the auto store, and I don't think I could've done it all those times without one.