Personally I like the have my brakes fairly snug rather than spindly so I prefer to get a slightly undersized brake and tweak it out to fit.
With regards to fitting a 115mm brake on a 120mm ski... have a look at the fingernail on your pinky finger. It's probably about 10mm wide. Then halve that width. That's the difference between a 115mm brake and a 120mm ski. And since there's 2 arms to the brake, halve it again for 2.5mm - that's about the width of a grain of rice. Given the manufacturing tolerances in a brake, you could probably just shove the brake on to the ski and the play in the brake assembly would "give" enough to get it on there. If not, a real quick tweak, with bare hands, would be enough to have it sitting nicely.
I've got 115mm-brake FKS on my Hellbents - the 132mm model - and the brakes aren't even significantly bent.