Putting the springs inboard allows the axle/wheels to put more leverage on the springs in a cornering environment and induce more body roll. Obviously your not going for corners lol. I would certainly run them inboard ONLY if your trying to runs tire under that bed you couldn't fit normally. That's pretty much all your gaining.
Is there a tire your after Tom? One that's not going to fit normally? It's much easier to weld on some new spring pads and leave the old ones in place than to remount all those hangers. Plus if you decide to move them inboard, the old stock plates are there and ready.
I wouldn't run a coil over on the rear. At least not right away. Initially, you just want the spring there to support the vehicle to the proper ride height. This means it should take 1-2" of spring preload to get that desired height. Being you have leafs already that will increase at spring rates different than what a coil spring will, it may cause you to have a hard time getting the back end to work right. A single leaf, pan hard bar, and sway bar as well as the coil overs would work better but a 4 link would be best and the most work to get to work for your truck.
I'm also not sure how caltracs will do on a single spring that has next to no preload in it as it's not supporting the weight of the vehicle. I have not looked into that at all. It may work but may need a lot of preload in the bars.
If it were me, here is what I would do. Figure out the tires you want to run and wheel off set. This obviously will determine how much tire will stick out the fender or not and where you need to mount the springs. If you can leave them outboard, leave them and just weld new spring perches on the axle and leave the stock ones in the axle. Leave the two springs on if ride height is where you want or use lowing hangers to get you down where you want. Buy double adjustable rear coilover shocks but no springs. This way you have the shock should you want to try a single spring and springs on the coil overs. A pan hard bar is a good idea to keep the rear end from walking around.
If you need to move the springs inboard, run the two springs, mount hangers so your at the right ride height you want, buy the double adjustable shocks without springs but look into the idea of an adjustable sway bar. At launch, with diesel torque, it will twist the chassis up when you get launching close to perfect and cause you to pull one way or the other. The sway bar will help eliminate it.
So basically, I can't tell you what the best way is for your setup but what i listed out is a good process of elimination.
