As a starting point, the stock axle for your 3200 is 61.02" wide according to the vehicle information kit from the GM Heritage Center (
https://www.gm.com/heritage/archive/vehicle-information-kits)
That is very close to a 67-73 Mustang/Ranchero. Not sure what other donors you are considering.
If you still have the stock axle installed, you can measure the clearance to the frame and the fender to see how much space you have for more tire width. The fender tapers a bit, so if you want to go to taller tires, make sure you take that extra height into consideration when measuring the width.
