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I would jack up both trucks and swap a wheel. Not knowing what wheels are on the 79, it is hard to say yes or no. Worst comes to worse, go have the tires mounted on his rims and vice versa.
4x4's generally have a larger center hole for the hubs to lock. Your big tires may not fit the 2wd without a lift.
I would measure the center hole of his rims to see if they are the same.