A couple things to remember...
When you change your
tranny fluid, half of it stays in the torque converter. So if you want to completely change it, you have to drain that, too.
If you are just changing the fluid that dumps out when you pull the pan off, use 4qts. If you are feeling manly and drain the torque converter, you will need 9qts. These numbers are assuming that the
tranny is from that year and its not a deep pan.
As far as should you do the fluid change yourself...
All you have to do is remove the bolts holding on the pan. There are like 8 of those. When the pan starts to get lose, the fluid will start to dump out, so be ready to catch it. Then there are another 6 or so screws holding on the filter. Pull those out and swap the old filter for the new one. Clean the gasket surfaces thoroughly and put the pan back on with the gasket that comes with the filter.
Now if you're feeling really manly and want to put new fluid in the torque converter, you can drain it by removing the bolts holding the flywheel inspection plate on. Then have somebody rotate the bolt in your crank pulley (clockwise) till you can see the drain plug in the torque converter. Be ready to catch the fluid when you loosen that one, too. I believe you can figure out how to fill the sucker back up from here. (close the drain screw and put the pan on before you pour the fluid in.)
And I would go with the regular, not the synthetic.
1966 Mustang Automatic Transmission Fluid 
