In situations of welding in difficult places I start out just a tack weld at a time. Takes a lot longer but ends up being easier. As for the measurements of your year of cab I don't know specifically. Not sure if an assembly manual would have that information. Since I stretched my cab I basically used the A pillar and front cab roof line and the door to move and make (what I felt) was an acceptable gap between panels.
You may want to just tack panels into place before committing to a full out welded in place to test fit your doors and rockers.
Your doing great so far. Just take an extra moment to check then go back to your 110 mph. Your flyin through this build
