[Big D]

I think 70 and before used 1/2" studs, 71 and 72 use 9/16". It looks like someone has put later model disc brakes on. It should not be a big deal to install larger lugs in the rear if thats what you want.

I found a good source for 9/16" lugnuts/studs is a local heavy truck dealer, they were about half of the price you would pay at NAPA.