I am about to change the fluid and filter in my Th350 because God only knows what is in there....

The trans shifts fine and seems solid but I want to do this a precautionary measure.

Before I drain it should I run some type of additive in it to help clean everything for a couple hundred miles?

Also while I'm in there I have heard you can drill something out for firmer shifts... kinda like a shift kit, should I go ahead and do this as well or just leave it alone?

And what is the best fluid to run in it? Any specific brand of filter to buy or not buy?