[dsraven]

when I have done some corner to corner checks in the past, by myself because thats the way it goes, I have used an old piece of light dog chain I had laying around. I took a bolt that fit into a hole that was symetrical to both frame rails on each corner and would also be able to fit into a squashed end link in the chain. this allowed me to fit the bolt into the frame hole to secure one end of the chain. I could stretch the chain out to get a corner to corner dimension without a worry of someone holding the tape measure differently on the other end from one check to another and the chain doesn't stretch like some tape measures do (fiberglass tape style). as long as I pulled the chain tight I could mark a link and compare to the other side. if I wanted to get more accurate I could fasten a piece of flat bar to the chain, mark the bar at the dimension for each side and measure the difference. this theory could also be used on shorter runs like what is recommended in the shop/assembly manual if working alone.
the frame drawings are a little tough to figure out because to get a given dimension you have to find a couple of other dimensions and do some arithmetic (thats math for the younger guys, lol). you prolly already figured that out if you have the drawing up on your computer, lol.