How to flatten the curve of a complicated STL I have been working with this STL file (a gauntlet for a prosthetic hand.) 


I need to flatten the curve at the top to fit the arm (basically just push the top down a bit), but the rest of it fits well, so I don't want to mess with the other proportions. I have been messing with selecting and grabbing the top and pushing it down, but this makes the inside of the gauntlet unsmooth with the pushed down materal. Any suggestions?

Here is the .blend file thru dropbox: https://dl.dropboxusercontent.com/u/95913396/extendedgauntlet1longer.blend
Select the parts of the object you don't want to affect and hide them with H.
Use projected proportional editing to adjust the model smoothly
Unhide the hidden parts of the mesh (AltH)