i want to leave my paintiac montana 2001 with out using in winter ( -20 to -40) for 3 months

I would set it on a peice of wood not card board though or a shelve just not on concrete

Your battery will keep a charge longer if placed on cold concrete. The cold slows down self discharge. Today's battery cases are not porous like the first batteries were. In the old days, acid would leak through the rubber case and cause a conductive path in the concrete that would discharge a battery. Back then, the wood prevented the conductive path because it would soak up and neutralize the acid.