I thought if you adjusted the bolts so there was clearance on both sides of what you named the frame then there would be some room on both sides so the cross member could move .
It sounds like even if you can get the front to move it still wont move enough unless the cross member was modified.
Sometimes if I need to take a ride on mower over very uneven ground I just put down a 19 mm sheet of plywood and take the mower over that like a bridge ,if the mower is only slipping in one spot it might be worth trying some plywood.
