Overdrive in Early A

Why would you consider the GM 4L80e series. That is a large transmission with lineage to th TH400. The 4L60/65 are electronic and upgraded versions of the 700 R4 with a 3.07 first ratio. But that is zalso likely a bit big for an early A body.
I would first consider a Mopar trans that bolts up to the engine that traces roots to the 904. Next option to me would be the 200 4R with its 2.76 first and .67 OD. This transmission was intended for GM 6 cylinder and 305 V8's. They were beefed up a bit by the General for use in the Regal T Types and Grand National turbo V6 application. The aftermarket got into the frey for the modded V6 drag racers putting out over 600HP. So they can be built to last and do not usually require a stand alone controller. The transmission is fairly compact so fitment is easier.