72 Demon rear wheel-tire sizes

Hello i have a 72 340 Demon with a 100% stock rear suspension. Id like to install a set of larger period correct 14 " Goodyear Polyglas tires and widened wheels to fill up the wheelwell E 70-14s are too skinny , how wide can i go and what inside offset do i need for spring and 1/4 panel clearance, thanks TX9 4-spd Demon.