68 Chev 3/4 4x4 overdrive trans swap

[Daimler]

Hello, I have a 68 Chev 3/4 ton 4x4 with a 350 and the stock 4 spd trans and stock transfer case. Both were rebuilt when I installed 1985 4:11 one ton diffs front and dually rear. (the original diffs were 4:56) I was wondering if a modern 1 ton 5 speed overdrive manual transmission can be fitted. (I remember a Torque Flyt auxilary tranny that could be placed between the existing tranny and the bell housing. It was 7 1/4 inches long and was rated for about 7500 GVW. It gave about 30% overdrive. I do believe another shift stick in the cab is great but I am not sure they are available any longer. Changing the drive shaft lengths is no issue.) I suppose it may be possible to swap the tranny and transfer case together to make it all a little easier, but I want to find out if a modern heavy duty 5 speed can be fitted to the original transfer case. Maybe someone here has done this swap? Presently I get 9 to 12 miles/gallon loaded.