Kim57 extended his all the way to back of cab to access with his tilt bed.
http://www.67-72chevytrucks.com/vboa...=283107&page=3
But if you cut the riser and just box around it ,should be ok the sheet metal is a lot more ridged than youd think,
But if you tac/weld in your frame that you are going to box it in first prior to cutting the riser should ease your concerns for any flex or movement.
Id cut some cardboard and try fitting your hand through it to get a good sized door opening.
OrrieG's door looks kinda tight, like id be leaving some meat on the surrounding sheet metal.