i've actually owned some form of mustang for the entire time i've been driving. my first ones were a couple of 67's, a couple of 68's and then a 74(that was a freebie).
the 60's stngs all had c-4's in them. the only complaint i ever had with those trannys, was the shifting when i modified the engines. that was easily fixable though with a little valve body work. not a single one of them ever popped out of park. i would be more inclined to think that that problem was somethign to do with the linkage, as ford had linkage rods, with a slot and locknut for adjustment.........not the chrysler k-cars that you couldn't park on even the slightest incline without it getting stuck in park.........