I have one, and as stated above, the early stock units, not to strong, but if you use the right parts, you won't have to worry about breaking them... It's the same basic tranny as used in the Mustang GT through 2010, they just made changes along the way including making them all electronic controlled, but the guts interchange, or atleast most of them...
Mine has a 95 Explorer case, early valve body to keep it manual shift, it has all the guts from 4r55 and 5r55 making it completely rollerized, 5 and 6 clutch drums, a double disc lockup convertor, and a bellhousing from a 2000 ranger..
My thought is, if it can hold up to a 3,800lb Mustang, it'll hold up to my 2,200lb Pinto with the same power....
People use to say the C-4 was weak and talked up the C-6