The brake cable holder I just removed and used the top one (backwards I think):
To clear the clamps I moved the tank backwards by re-drilling the aluminum plate holes. To keep the tank up (left valve cover is a contact point) I spaced the rubber mount up with a few washers:
Seems to be working, the clamps clear and there is still clearance on the valve cover:
For the seat I tried just cutting the ribs underneath to allow it to bend, but that wasn't enough so I pulled some staples and cut the plastic under the vinyl:
Bends easy now, and I stapled it back up so the vinyl is uncut.