With the 2.5” brakes my notes show you should have 4-23/32” from back of hub where drum mounts to backing plate flange.
Dual wheel type hubs on that single wheel axle results in 2-19/32”. So you move the flanges away from the wheel another 2-1/8” each side to use those brakes again.
You should investigate if the 2.5” and 3.5” brakes use same backing plate. If so you’re well on your way to having the larger brakes, if you want them. The shallower of the 13x3.5” drums uses essentially same measurement (as 2.5” brakes) from back of hub to backing plate flange, 4-11/16”. This 1/32” variation could be acceptable in manufacturing, I’ve only had a few axles to measure, not enough of the same to compare.
Also, take your own measurements and do your own calculations in case your situation is different or I messed mine up.
