I thought it would be a little too lucky for a jeep to have the 65-6 trans,and it is easy to mistake the two bcause they look the same at a quick glance.
I've heard Jeep did use the Nailhead case ST400/TH400/whatever400 behind some of the Jeeps in the 1960's-70's.
The Nailhead's trans 'bell' is shorter than other applications, so it was well suited to using an adapter to fit other engines.
I have some notes copied from the HAMB board that states the Buick trans was used on Jeep Wagoneers and pickups from 65-72. The Nailhead trans was also rumored to have been used in some Jaguar and Rolls Royce!
This applies to the 3 speed '400' trans only. I don't know if the 2 speed '300' trans was ever used in non-Buicks.
And Wheelz, there is even mention the Buick 350 engine (when installed in a Jeep) did use an adapter plate to mate to the Nailhead-style '400' trans!.... I don't know why they didn't just use a BOP TH400 trans, it may have been a licensing thing.
Just verify it's a 2 speed '300' trans with the correct output shaft (count the splines).
Good luck with the purchase, hope it works for you.
BTW, does that (Jeep) trans have the switch pitch converter???