Conti and Bontrager do 48 mm presta tubes for 26 x 1.5, I have them on my Reynold 32mm carbon rims, with about 15 mm sticking out. Or get valve extensions for about 7 quid on ebay, CR etc...
The new wheel is likely to create you quicker up the mountain and all over the sides,just in here we are at mud year, some beefy street bicycle on the dust tires.if you think a little bit of air time is the best way to sleek out the challenges.