Bryan,
Mustang Monthly has a nice article about the C4 in the January 2011 mag. the C4 was produced from '64 thru '82. It was replaced by the C5 and updated with a "lock-up" converter. It was replace in '86 by the AOD. When I rebuilt my '68 C4 (used in my 64.5), I used the beefier components of the later models as I was pushing anywhere from 350 to 400 RWHP. Personally, I used a Trans Go kit with the larger "C" code server. Does you tranny still have the tag? This is where most of the mfg ID stuff is located. I'm assuming your tranny pan is square, as well.
Probably not much help, but, there is a lot on the web.
Good Luck!
If it has a tag this will tell you of the original application. http://mmerlinn.tripod.com/trans/fdc4f/fbec4_s0.htm
The casting # is on the housing above the shift shaft. 70 is when they changed valvebodies. I would use a TransGo kit.
The casting # is on the housing above the shift shaft. 70 is when they changed valvebodies. I would use a TransGo kit.