It looks like Shimano offered their 7 speed mountain freehubs in both a 130 and 135mm spacing. According to the parts diagrams, the only differences are an extra 5mm spacer in the 135mm hubs, and different axle lengths. So - try hunting down a NewOldStock 7 speed mountain freehub of either width. If all you can find are 135's, you can narrow those. Or you can do as mentioned here and spread the frame.