If it was me.. I’d first make it squared up by finding similar points on each side and cross measure them so each match in measurements. The build braces to secure them. Next I’d make sure it’s absolutly level. Do the secure (screw) the fire wall first because the floor rests on to of the it’s lip. Secure (screw) The the floor section (wheel well j area next. Then lay the floor pan next laying it above the other panels lips... check the alignments.. this will take time be sure everything is square. Once it fits and measures right start ( having fun... can’t teach over the net but with patience..It can be done nicely.. but this is no amateurs job to say the least.... :0....... may the force be with you on this one...:0