They don't seem to be consistent. Can I just make the tops of the mounts flush with the top of the frame rails?
The only true way to know where to mount them on the frame is to put the body on there and then screw the mount in and then tack it in place.
They aren't even close to the originals, so before you cut them off, make sure the spacing is correct... from the frame to the center of the hole you are cutting off and then the one you want to weld on.
Make marks on the frame for center lines, etc.
OR, just remove the bad section and cut the new stock down and make it fit
If you put them too high or low, the body mount bushing might be too squished in there or not enough room to take up the slack.
cb
cb