Yeah, looks like you can easily shave an inch off the long end of the gussets, and a half inch on the two shorter ends each.

What makes your bumper mounts weak is that there is nothing supporting most of the front where the bumper makes contact. There is a large hole in between the mount, which is likely where something is going to cave into. If you still want to use that method of mounting, I'd suggest putting some wood or another light but solid material inside the gaps to prevent them from bending inwards. Another option is to make a rectangle around your base using 1x1x.125 aluminum to add bumpers to and to increase mounting space (especially if you're not welding, mounting space for gussets is important, and it looks like a lot of the space on your 2x1's is taken up). Here is an example of the surrounding 1x1 rectangle: http://media.team254.com/2012/08/drivebase.jpg .