I don't think you need to res-pace for 126 mm. I just finished rebuilding my '87 Schwinn. Before this I never had occasion to consider the frame spacing - I just rode it the way I bought it.

But this time around I checked and guess what? 120 mm chain stays and 126 mm 6 speed freewheel. Came that way new. I'd been spreading the chainstays with my hands as I installed the rear wheel and never considered that this wasn't normal. Thousands and thousands of miles and the hub is still fine.