Should I install a moisture barrier behind drywall in house with T1-11 siding?
Increasing your wall thickness to facilitate 6in insulation is a fine idea, can't hurt. A vapor barrier of 4 mil plastic with taped seams should always be installed between the insulation and drywall. this will stop moisture from the outside from penetrating the back of the drywall and conversely, humidity from inside collecting in the insulation. T111 siding is notorious for not being a tight exterior sheathing, especially when installed without a backer such as OSB or plywood covered with a layer of Tyvec. The interior of the wall will vent itself and ambient moisture in the wall will not be a problem in most cases, as long as there is not any water infiltration into the cavity, you will be fine.