The new tires need to go up front, per what I was taught. In inclement weather the front tires are under greater forces than the rears especially when braking, thus we need the greater wet traction up front.
The industry has flipped on this three times I'm aware of and might change tomorrow. Plus in 7,500 miles or so you're going to rotate the tires, what then?
The worn out tire against the new tire on the drive axle isn't a concern either, that 3/8" difference in tread won't make a significant difference in revs per mile, not enough to hurt the diff. Now on a dually the new tire can't go against an old tire on the same side, the new tire will get wiped off quickly, experienced that one first hand.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10