4runner dude

well i rolled the 87 4runner so while im waiting on new fenders and doors im adding a few mods and had some questions.

1st mod: dual batteries using painless wiring p/n 40102
question, where to put battery #2 and what batteries to use?
2nd mod: dual t case of RF1A. i have 2 cases already and ordered the adapter from trail gear.
question, how do i set it up, do i need anything else?
3rd mod: making half doors out of the dented doors.
question: where should i cut the door level at thats easy to cap the top afterwards?
4th mod. new 15 gallon custom built tank
question. should i mount this where the tire carrier was? or in the bed?
5th mod. on board air. got a cheap compressor and 7 gallon tank.
question: where should i mount them both?

my main concern is the dual tcase questions. i can figure out the rest if neccesary

thanks