When my master failed I would be stopped and gradually lose brake pressure and the truck would start moving unless I pressed harder.
Here is the FSM booster test...
I measured a new rear floor and mid bed panel (real steel) and they both were 40 7/8"
From this image it appears that the factory dimension is 1015mm which is more like 40" but as you found they are typically closer to 41".
Definitely welded by PO. All components of the frame are rivited togther from factory with the exception of a couple brackets in the front frame horns AFAIK
Was thinking my truck was missing the light but this is how my '67 is, hole under that is blocked by the heater but no hole in the dash. I never even knew that hole was under there. Glad I don't need to find a vader light!
There is definitely a nut plate that the bolts screw into and have some play. You can remove/loosen the lower hinge bolts which are much easier to get to and see how much play there is before getting all twisted and scraped up getting the top ones.