51:
The wood block height plus the cross sill height(including its little metal plate where the bed to frame bolt goes through) should be the same as the rear sill mounting bracket height. That way the bed sits flat and level with the frame and is not put in a bind by the bed-to-frame bolts that go through the cross sill. If your frame is distorted or bent you may have to adjust, but usually 2-1/8" works.
Mar-Ktech
