The C-4 is a great automatic but yours probably need a rebuild. The Pro-5.0 Mustangs run beefed up C-4 automatics behind 1000 HP Nitrous monster motors. I think Jegs and Summit sell reworked C-4 trannies. The C-6 will be beefier but it is a HOG very heavy and it will suck more power from your 302.
Here is a link to Jegs
http://www.jegs.com/cgi-bin/ncommerc...06&prmenbr=361
If you want to run a real low rear gear(3.55, 3.73, 4.10) you might consider a reworked AOD. This will give you great low end and still give you livable rpm's on the highway.