For what it's worth guys, I have the same cab on my 1864 and do not have the fender mounts installed (partly because it didn't come with them
). I just have the two rear braces that bolt to the frame and the mount points on the footrests; so far, no issues.
Edit: Ok, so I lied. I have the mounts that are supposed to secure it to the fenders, just never drilled the holes and bolted it up. Haven't seen a need to yet.