Your tire size is pretty limited due to the dual wheels. 235's will fit on the back, but they will rub each other if you put too much weight in the bed. You really can't get away with anything bigger without doing something scandalous, like spacing the wheels slightly further apart.
Good luck finding a lift kit for it. I think you can use most of the parts from a 3/4T lift kit and fab any other parts. Surely someone on this board has already done it.... - Mr Diesel
'76 C30 Crew 6.2