73 Duster Mini Tubbed - How big a wheel?

73 Duster Mini Tubbed springs moved to the rails - what size torque thrusts should I get in the back? It has Mickey Thompson 29x12.50x15 right now with stock a body with 8 3/4. Anyone know backspacing and rim size and width maybe in a 17" torque thrust? I know this is a shot in the dark. Thanks.