I never got the link yesterday since my computer was off all day and this is the first time I have seen it.
I made (modified normal ones actually) adjustable jackscrew jackstands specifically for this purpose. the easiest way is to buy the cheap tubular ones, drill a 3/4" hole in the center of the head and use industrial threaded rod and nuts to fashion the adjusters. Weld on nut to the top and then use another to snug the rod. I used some 3" square 1/8" plates I had laying around and welded nuts to them for the part to rest the car on.
The best place to level the car is measuring at the front and rear of the rocker assemblies as the chance of them being out of square is minimal. If there is any question take it to a frame shop and have it squared first....this is what I did. You will have to fashion some type of trammel bar that you can attach to the undersine lip of the rockers to hold the level or you could cheat and just use the level on a beam across the rocker on the top at the front and back of the door opening. I used the frame rails as measuring points for level but my car is solid, rust free, and just came home from the frame shop and I know it is dead on. You are replacing the frame rails so I suggest you use the rockers to set level and then get the frame rails level and paralllel to them.
I hope this helps. You can IM me if I can help you in any way.....they are in my profile.