No truck... yet but looking with plans

[sandrider123]

So I do not have a pick-up yet but I am lurking I have been reading all the build up threads. Kind of have a plan devised for what I am looking to do and what I want. basically looking for a 73-91 crew cab 4x4 The plan is to swap a 24V cummins into it I wont cut it down to a short bed becasue I need something that can tow a 5th wheel every now and then . I am also looking to put an automatic in it but I am not sure what I ahve for options I want to find a way to put an allison in it but I am not sure what i would need for a controller and mounts. Interior wise I am looking at going through and replacing the seats with something out of a newer pick up, Auto-Meter Guages, and probably a new carpet kit or possibly bed liner for a floor. For other amenities I am looking to install a nicer stereo, probably some dakota digital LED taillights as well as HID headlights. A few questions I do have aside from the previously stated transmission questions. is transfer case what s a good option to put behind a cummins what will probably have light mods (exhaust, intake and a tuner) and what is the best way to update the steering on these older K series trucks?