What would be the best way to make the rear frame wider to make it fit a wider hub? I am going from 110 to 125mm. Would it be the same way you widen the rear for a wider tire? Spring clamp and some heat?
get some all-thread at Home Depot the same size as the hub axel.
gets some nuts and washers.
put them on the all-thread with them inside the drop outs, and with a wrench, run one of the nuts out to spread the chain stays.
there will be some spring back, when you take the all thread back out, so, check results with your hub and repeat if necessary.