Re: 69 vert underbody repair process
You have a little fun ahead of you. My 69 vert needed pretty much most of the items you listed. I completed it without a jig but did it piece at a time by myself. I had no previous body work experience.
Suggestions that worked for me:
Level the car and weld braces across the door frames and make an x spanning from each rear panel to the opposite side kick panel.
Measure everything, take notes and keep a level handy!!!!!!!!!!!!!!!
Keep doors and trunk lid on and adjusted.
Start with the full floor. Remove old floor but make note of the location of the vert only K brace and bolt on the X brace in if you have it. Screw and clamp new floor in place.
Move on to the Toe boards. Weld to firewall but screw to full floor.
Move onto the inner wheel houses. I removed and replaced them 1 at a time leaving the quarter and outer in place. It is a little more difficult but it makes the alignment easier. Screw in place and weld later.
I moved onto the full floor, cross rail and frame rails. I removed them all as 1 unit dropping them out keeping the quarters, tail pan and wheelhouses in place. The bottoms of the quarters need to be pulled outward so the assembly can be removed in 1 peice. Set up the old assembly on horses and use as a template to make the new assembly. I added the trunk drops later. Screw together new assembly next to old one and once all dims match weld the assembly up. Sneak the new assembly up into the car and screw in place and bolt to tail pan through bumper bolt holes. I had to use a come along to pull the frame rails tight to the cabin full floor. Once fit is completed start welding the truck floor assembly in place. Weld to inner wheelhouses, rear seat divider, cabin full floor.
I did not replace my rockers and it would make sense move onto those at this point. Others my have better ideas on when to replace the rockers but make sure you keep braces in place.
Next cut out 1 quarter at a time. Just cutting away enough at the wheel well so the outer wheelhouse can be removed. Fit outer wheelhouse and screw in place. Once set move onto removing the rest of that quarter. Carefully remove quarter keeping tail pan in place. Once quarter is fit to trunk gutter, tailpan, trunk lid, rocker, pinch weld, door and outer wheelhouse remove it and weld the outer wheelhouse to the inner wheelhouse and vert brace. Install trunk drops and put the quarter back on. Weld it up but screw to tail pan. Move onto the other side.
Once quarters are complete install new tail pan.
At this point the car went onto a rotisserie to finish welding and clean up welds.
This worked for me and my gaps turned out great!