Clearance between header and ground ???? 1971 A Body

Your concerned about the headers hitting the ramp/floor at it's apex. So obviously the longer the ramp the less obtrusive the angle when the headers pass over it. As a quick fix you could progressively block on top of the ramp where it intersects the floor in the tire path. Start out with a single flat 2x6x3' then double it for the next 3' as you reach the top of the ramp. Reverse the layout going into the shed dropping back down to the decking. 2-2x6 would give you an extra 3" going over the apex, that combined with the actual ground height of the headers should offer plenty of clearance..