Mt 325-50-15

I measured a mt sportsman pro with a sec with of 13.1 on a 12 inch rim with the tire on the floor an a strait edge it measured 13 an about an 1/8. I guess that's why I thought I could get away with 12 inch rims. The tires are still in transit maybe Monday I should get them, I'm going to check it out carefully before I mount them. I'm hoping they fit but if not I can send the rims back for another size if it's to close. I put the rims on the car, held on with a couple of lugs not tightened down just by hand. And with a strait edge on the face of the rim with the strait edge all the way up to the fender there's a good 5/8 space. And about the same on the back side of the rim. Got my fingers crossed.
Explanation on the sportsman tire, was a local guy was selling them, they where new tires but the draglite rims new days where long gone, I didn't buy them. They where on a 12 inch rim though. He wouldn't split them from the rims nor lower the price. Im hoping the drag radials sidewall flattens out as the rim gets wider, I'm hoping anyway. But I guess I'm going to find out. Thanks..