You won't have to do anything to it, internally they are pretty much last forever indestructible. The only thing you need to do is put new front and rear main seals on it, replace the o-ring on the fill tube, replace the flying saucer-shaped modulator on the back, and put a new pan gasket on it. All that stuff is CHEAP and just ensures against leaks, except for the modulator, which is what makes the tranny shift.
Tips: Buy the "deluxe" oil pan gasket, not the cork kind, they work much better. Do NOT put sealer on the gasket. Make sure the pan isn't dented/warped where the bolts are. Don't overtighten the bolts, only torque them to the ridiculously low spec of 7-9 ft/lbs. When you unscrew the modulator there will be a 1/4" diameter x 1" long tube inside the hole, DON'T LOSE IT! That tube is what the diaphragm inside the modulator pushes on to make the tranny shift. The fill tube is very difficult to install and get seated into the tranny when the engine is in the car, try to reinstall the engine/tranny with the tube already inserted. It MUST be pushed in until the bulge/ring on the tube is tight against the tranny. DO NOT hammer on the tube to try to get it in. If it isn't all the way in it will leak and the tranny will suck air when you go around turns and slip out of gear. A 1/4" ID rubber hose goes from the modulator to the intake manifold, the suction from the manifold is what makes the tranny shift. Put a new hose on to assure no leaks, if it leaks the tranny won't shift. There is an "on demand gear kick down function" that drops the tranny one gear when you squash the pedal to the floor. There is a "kick down cable" that gets yanked that does this. They are often broken where they attach to the gas pedal. You adjust them to only get pulled just before your gas pedal hits the floor, adjust them to get pulled before that and the ball on the cable gets ripped off. They aren't really necessary, your car will most likely downshift on it's own without that cable being pulled. Make sure to unbolt the tranny gear selector lever from the tranny when you drop your engine! People forget to disconnect it and the pivot ball gets snapped off.
There is no need to rebuild these trannies. Most people take them out and install 4-5 speeds, so there's lots of used ones out there that people will give you for free if for some reason you need a new one. I have NEVER heard of one of these trannies breaking or wearing out in the 40 years I've been driving GT's. I have owned and heard of HUNDREDS of 4-5 speeds that were worn out. It's your first car, I take it, so keep it simple and keep the auto tranny and check tranny problems off the list.
Often, you don't need to replace ANY of the stuff I just mentioned, no matter how old the tranny is, it will work just fine.