OK, so check my math.
Current: 61" wms, 10" wide rims w/3.75 bs, 12.50" wide tire
So, 61 + (2 * (10 - 3.75)) + (2 * ((12.5 - 10)/2)) = 76"
WMS + (rim width - bs * 2 rims) + (tire width - rim width to get how far the tire sticks out past the rim divide by 2 to get extra width on...