If you used Devcon plastic welder, superglue, white glue, etc... getting it apart is really easy. Throw them in the freezer for an hour and flex them a little and they will pop apart. If you used ABS plumbing cement you sill have to cut them apart (good luck).
Since you need them shorter you have a golden opportunity to make the contour more accurate at the same time. If you do this leave the middle strip length alone and start cutting the contour first. Notice the steep angle on the outside curve, and how deep the inner cut out is.
For shimming them so they are bigger at the top and smaller at the bottom. First figure out how much you need to trim off the bottom if you need to. I had to trim about 3 inches off the left just to get them to match. So I cut a wedge taking about 3 inches off the bottom and none at the top. Then add a wedged shim to give you however much you need at the top while keeping the bottom nice and contoured, and reinforce from behind for a stronger hold.
I'm working on a tutorial on a better way to shim thighs as we speak. I've been helping a local recruit with this very problem. Hopefully I'll get it up in the next week or two.
Good luck.