We've cut plenty of these racks down as well. We used to source all our MA gear from $#@d and they would stock the full length rails which are nearly the same price and we could just pick them up and cut them down to size. 

It sounds like its going to be a pain in the butt, so be sure to add in some extra hours on your estimate. 

Honestly, at that size, I would be doing a CFR rack or a small 12U from snap, two that you can just pull completely out. Mabye have a table or something that you can slide them out on to and move off to the side.

Actually with the slide-out mechanism taking basically 2U, you only get 10U of space. Go with a CFR and you at least get 11U and will most likely be happier. Since we started using this method, I can't see where I will use an slide out on rail style rack again.