ECE 4/6 Owners pulling the trigger. Have some Q's

[72tutone]

So I'm ready to pull the trigger on the ECE 4/6 kit. I have read numerous threads about meausuring prior to have a baseline. Right now my bed is off and I'm doing other work on it in which it wont be back on for sometime.
Is this going to be a showstopper for me? I can measure the front to the front fenders, but not the rear.
Any suggestions? Thoughts?
Should I proceed in purchasing? I would think actually have the bed off would make this install easier.
I just dont want to shoot myself in the foot, as I have saved for a long time to buy this kit and dont want to make a mistake.