wtp over drive trans behind 340 in a 68 a body what can i do

I don't know of any automatic overdrive transmission that will bolt into a 1968 A body Mopar without modifying the tunnel & torsion bar crossmember. Maybe I'm wrong? :dontknow:

I think the Gear Vendors unit would probably be the closest you would get. You could try clicking on the Keisler link on the right & see what they have in the way of an auto.