Need to rebuild the automatic transmission 1996 GT
 

I need to fix the automatic transmission in my 1996 mustang gt, I dunno which trans it has, it was acting like it was going over rumble strips, now when i hit the gas it revs and takes forever to go any where,

transmission had no oil in as it was leaking into the coolant due to the oil cooler, mechanic had me believing it was a cracked block after replacing the head gaskets, i flushed the coolant like 30 times in a month, which drained all the transmission fluid....

I think i need a Auto Trans Torque Converter??? or is it something else? what parts do i need?

Should i stick with motorcraft parts? or is there better aftermarket

What upgrade could i do while the trans is out? I drive hard and plan to add a tubro

You have a 4R70W.

As far as relacements, depending on budget search junkyards (sometimes they have units already removed and occasionally offer a warranty or guarantee of some sort), local shop for rebuild, ebay for used or rebuilt unit, etc. These transmissions are very common and were used in several other Ford vehicles such as CrownVic/GrandMarquis/Towncar. They were also in F150's but I am unsure if there are any internal or external differences, and as far as a used one goes I think it would be preferable to source one from something that did not potentially get used for hard/severe duty.

As far as making it better, type "4R70W J-Mod" into Google and start reading up on that. It's basically a shift kit that was designed by the very Ford engineer that designed your transmission. You may or may not absolutely need to replace the torque convertor (chances are you probably do), but if it were me I would put that on your list as well. Where it can get tricky is if you opt to go with a unit with a higher stall speed than the stock unit; generally speaking something with about a 3000 to 3500 rpm stall speed works well for a car that will be primarily street driven (consider the stock unit is about 2500) but if you are serious about going turbo in the future then that may need to be taken into consideration as well.