The early Toploaders had the same 25 spline output as the T-10's. The later (66-up, not sure when the switch was made) Toploaders have either a 28 or a 31 spline output. Beware when choosing a Toploader as the 390 specific Toploader has a shorter input snout that does not work with a small blockk bell, unless you use a thicker pilot bearing/bushing (Boss 302 & some of the early small Bronco's use this) Also verify that the front bearing retainer's outer diameter matches your bellhousing's hole. The early Toploader is easy to spot, it's got a completely different bell housing bolt pattern.