What you,ve heard about not putting dextron fuild in dodges is correct but only for the late model overdrive transmissions.
Dexron will work just fine in the older 3 speed trans and was the recommended fuild back then. You could run the atf4 in it as well but i wouldn,t mix the 2 types together .
Changing it is fairly easy but can be messy if your not careful since they didn,t come with drain plugs in the pan.
You sould also check to see if your torque convertor has a drain plug ( most of the old ones did ) and drain that to as well as adjust both bands . Kind of to much to type out step by step right now but it,s all pretty easy if your handy .
