I learned the hard way NOT to use ATF +3/+4 in two of my past cars:
1) '71 Charger SE 383 hp with mild rebuild (425 HP gross), 727 and 3.23 one-leg ... burnt tranny up 500 miles after broke in the engine , went to Hughes TC and trans rebuild and Dexron II and no more problems.
2) '74 Dart Sport 360 with mile rebuild (298 rwhp) and 3.91 sure-grip and 727 ... burnt tranny up within 1000 miles of engine break-in , went to Mopar 175K TC and trans rebuild using ATF +4, had to REDO trans within 400 miles as it already had "black death" in the fluid!

Both trannys were serviced @ Chrysler dealership I worked at prior to engine rebuilds, both had ATF +4 put in 'em and toasted both even with tranny coolers in already! Rarely power-braked either one as they both broke loose from a dead stop quite well ... just my worth.
No old in new, no new in old anymore for me ... Imperial doing great w/Dexron in it.