Extending torque arms?
I don't know about cutting and re-welding, suppose it's doable. I got a set of adjustable from CCV, boulder off road also makes a nice longer fixed set. Just FYI, I think in the end to get proper clearance for 315's we had to move the axle forward about 1.25"-1.5" from stock, but doing so we ran into clearance issues with the track bar, so that had to be re-worked along with a custom mount to bring it back into alignment, also put on new bigger sway bar with end links, ride is greatly improved, esp with Ramsey's kit. Good luck!
'03 Ford E350 7.3L Diesel
(de)SMB'd Custom RB-50
Quigley 4X4 w/Deavers & Agile Offroad's R.I.P. package
CCV High Profile Pop Top