Offset Shackles

To use those shackles, you also need the corresponding hangers which is part number DCC-4120077 in the Summit catalog. Plus to do it right you will need to move the spring perches in on the axle housing about 3/4" on each side.

Shackles - $58
Hangers - $129
Spring perches - $13
welding labor - $$

The maximum tire width would be about 2 sizes wider than the maximum for the stock spring location... that assumes your wheel backspacing increases the same amount.

Or you could get the Spring Relocation Kit and get approx 3" on each side for $120 plus welding labor. Combine this with a mini-tub and you can fit 12.5" tires with no problem