What tires and width in the back of my stepside??

[1974chevystepside]

Hi guys well i finally got her up and running! Magnacharged 355 . any way the back tires are MT Sportsman 29x15x15.5...wide tire..it looks badass.. but are they too wide for the amount of weight in the back??? plus the rubber is 5 years old on those tires. it breaks loose at 40 mph easily! so any recommendations?? i am looking to got with a 30 inch tall tire as it is 4.11 gears with a th700r4...loooow 1st gear! its runs like a scalded ass monkey! Thanks guys!!!