You should be able to extend your spare carrier bar downward, assuming the bolts fastening it to the crossbars are long enough.
Or you could buy a new 6.50-16 tire HERE. I got 5 of them for my truck at just under $100 each. Had to do a jump through a few hoops to get them to me in Alaska. They wanted $3000 to ship them to me directly, but offered free shipping to the lower 48.

Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.