The left (driver's) side is 4 5/8" wide & the right (pass) side is 3 1/4" wide.
These measurements are made at the top of each and it appears as though they may taper a bit wider at the bolt holes, but it's hard to tell for certain. I tried to round up on my measurements simply because the tops have a fairly heavy radius, so I wnted to be as accurate as possible for ya.