Just about ready to do the pinch weld blackout on my car. Will also be blacking out anything that hangout below the pinch weld, like the front frame rails torque boxes and convertible seat support. Do the rear frame rails get the same blackout and how far?
Rear frame rails in the wheel well and forward of that would not typically be in the spray path of the paint application. You may find some "blow out" in the front edge of the wheel well onto the sound deadener and some (wheel well edge works as a mask for much of the area from that angle) of the frame rail right behind it for maybe a 6" round spot at best. This would have happened when the gun/spray reached the end of that front pinch weld surface and is often repeated at the rear of the wheel well when the sprayer started up again. Didn't happen that way every time - just depended on when the spray was triggered and what angle the spray head was aimed.
The frame rail behind the wheel well can some some overspray/direct application depending on the angle of the spray. This would be visible on the bottom and outer surface of the frame rail. If doing a later car with a dark batch undercarriage at the rear the overspray may go unnoticed. An example of this is picture labeled U-12 in the thread titled 70 GT500 - Unrestored Picture Thread
Im also wondering if the rear wheelwells also got the blackout at this stage at Ford . I would think so as it would give the car more finished or presentable look.
For what every reason, doesn't always have to make sense, Ford didn't black out rear wheel wells. Plenty of original pictures with bright light colored rear wheel wells (same goes with Dearborn front wheel wells) showing nice coats of body color visible to anyone to see.
After 50 years most have the paint coat worn or fallen off, at least top layer, covered with dirt and grime from the decades of use.
Looks like allot of body color paint got covered or scrubbed away from the outside surface of that rear frame rail in your picture. Top most picture in your second post