Just a little expectation management, that RC 6" lift kit is 6" cranked to the moon, otherwise it's 4" which I think is still enough for 35s.
If you're doing a filter, go whole hog and purge the entire trans. Do the filter and refill by 2 quarts over. Pull the return line at the trans, stick a hose on it, and run it to a 1 gallon jug. Idle the motor until the jug is full. Add a gallon and repeat. Fill 3-4 jugs for a squeaky clean transmission. Don't listen to all the paranoids out there who tell you this will kill your trans, it won't. Also, if you have the original ribbed rubber gasket in there, reuse it. It's superior to any aftermarket gasket.
If the diff is out then by all means do the oil pan gasket. Too easy. Make sure to glob RTV up in the corners of the main caps at the block and the gasket or it'll leak. Don't be shy with the RTV either. On the other hand, don't go full retard either.