What all is involved in changing the transmission fluid in a C6 Transmission in a 1973 Mustang?

It is not hard at all. remove all but one pan bolt and put your catch basin under the whole pan. Use a hand held socket to remove the last bolt. Just slightly loosen it and see if the pan comes down on its own, if not, use a flat head screw driver and gently tap it between the case and the pan until it releases. once you break it way, continue to slowly loosen the pan until the fluid comes out. If by some remote chance this is the first time it has been change you will find a funny shaped plastic item that looks like a top. Throw it away, it was a plug for the filler tube from the factory. Line up your torque converter until you see a pipe plug sitting at 6 o'clock. pull that plug out and drain the TC. put a dab of loc tight or sealer on the pipe thread when you put it back in.

Take the pan after you have cleaned it and use a ball pean hammer to pound out the bolt hole. Lay the edge of the pan on the edge of a surface and gently tap each hole so it is flush with the gasket surace or even a little lover than the metal surface. When you install the pan bolts becarfull not to crossthread. start each one by hand and do not, I repeat do not use air tools to put the bolts back in. Tighten them by hand and tighten them to the torque spec for your car. If you can't find them, watch the gasket and tighten them until you just see the gasket start to move out. Stop there. Move around the pan in a cross pattern and then go around with a final snug one bolt at a time to be sure you did not miss any.

Pretty simple, just drop the pan and replace the filter and gasket. If you leave a couple of bolts in one corner of the pan about halfway in, when you break loose the pan it will be a little easier to catch the old oil as most of it will run out the opposite side. This method will change the fluid in the transmission, but not the fluid in the converter. Usually this is not an issue unless the fluid is water contaminated or extremely dirty.