If it were mine, I would leave it at 135. If you are using a modern hub there are plenty of 135mm hub to choose from. If you are using a vintage hub, buy a long axle and add some spacers to the left side. This will allow much less dish in your rear wheel and should make the wheel stronger and last longer. Back in the old days when this bike was new(er), Phil Wood was the only way to fly, because the axles were immune to bending. With a little patience you could probably dig up a vintage Phil Wood 40 hole or 48 hole hub that would be a perfect fit.
Brad Luecke, Columbia MO, USA