By all means cross brace like Drew suggest. I just went through the same thing and found no easy way. Did a lot of measuring, remeasuring and guessing. My door post was damaged from the bottom hinge down. All those step fit aftermarket pieces are necessary but don't expect them to fit all that well. Tack it together and check the door fit. I didn't get it right the first time and underestimated how much the floor drops at the outboard end so the door drug at the bottom due to where I located the step flange on the rocker to door post replacement piece. Was glad it was just tacked. The other thing was locating the fender flange plate. get that wrong and you'll find the fender doesn't fit when its too late. It was a lot of trial and error and left/right comparison but i've dry fitted the fenders and doors, everything seems to fit. Probably not much help other than knowing your not alone. Good Luck with it.
Bob