Plenty of boards available around that length with a mid 250s waist. I've got a 156 with 255 waist, 157 with 251 and 161 with 254. All are fine with a 10.5 boot.
The Process could work, though a little on the soft side for powder and back country, could be worth going with the 159 to make up for the flex, coming from a 166 going a little longer won't be an issue.
For the style of riding your describing, a Ride Highlife UL could work well. A 158 has a 252 waist, you wouldn't want to go any shorter for back country riding, and UL stands for Ultra Light so ideal for climbing with.
Libtech TRS as mention is 253 in the 157 or 159, though Libtech run on the heavier side.
Burton Juice Wagon in 157 has a 254 waist, though may be a tad soft for back country, given your weight could be okay. Depends how much softer you're wanting to go from the Morrow.
Rossignol One Magtek in 156 is a 253, 159 255.
If you're okay with spending a bit more, Neversummer Raptor in 159 is a 253 waist, or slightly softer Heritage in 158 is a 254 waist.
Just a few examples of many options. As you get more into back country, a split board would be a wise investment, though sounds like you're a while off that yet.
Last edited by Phedder; 10-25-2014 at 09:54 PM.