Trans swap from T400 to Overdrive

[Eyegore]

OK this is transmissions for dummys, me being the dummy. If I want to change the exisitng T400 to a overdrive unit, whatever the 4l80 is, but without a computer to run it and w/o changing tailshaft and cutting down the driveline but only changing the selector to in OD, can it be done? And what is the most inexspensive way to do it and what year of tranrmission should I be looking for?
This is for a 82 3/4 ton w/ 6.2 diesel

Tom