One issue is the length of the bar depends on where I measure it. It varies from about 39-1/16" to 39-1/8" around the circumference by tape measure. At precisely 2" OD measured with calipers, that's approaching 2 degrees perpendicularity tolerance on the cut. Unless they were cut with a hacksaw by hand, that's pretty loose. The measurement between my end plates is a bit difficult to take without something adjustable and rigid with sufficient length handy, but by string measurement it seems to be 39" at most, perhaps less if I was stretching the string. This would probably also vary depending where on the plates I measure if I could do so accurately.
If the bar was designed to be tight fit as you say for only a few tested cars, naturally many cars will fall on the smaller side of that range and be too tight. Therefore something like this must be designed for worst case min with tolerance for a larger span. I don't know what Ford's datum scheme was for this area of the whitebody, but I can imagine 3mm (~1/8" as I estimated) tower-to-tower after upwards of thirteen years of fatigue could be possible.
Thanks for the measurements.
Cross brace was cut with a miter saw type chop saw at the supplier. The cross brace was to be held to 39±1/8". All cross braces where hit on the ends with a grinder to remove any burrs (which may be the reason for the angles you see), but will not effect the function of the RSTB. Based on the trunk measurements (39.875", 39.625", 39.6875"), I figured I'd take a low end target of 39.5" to be safe. So lets look at the tolerance stack up...
High end tolerance of trunk width
39.875"
Low end of tolerance of cross brace
38.875"
Bracket thicknesses taken into account (12ga steel, 0.1046")
2x 0.1046" = 0.2092"
So, 39.875 - (38.875+.2092) = 0.7908" clearance
Low end tolerance of trunk width
39.5"
High end of tolerance of cross brace
39.125"
Bracket thickness, assume decent tolerance (most likely excessive)
0.25"
So, 39.5" - (39.125+.25) = 0.125" clearance
Then there is room for adjustment due to the oversize holes in the cross brace and the slotted brackets as well. However I don't feel like going into the detail on the positional tolerances and stack up on all of that as well here...
Bottom line your cross brace doesn't fit. It appears your car is a lot shorter in width then the ones I measured. Hind sight I should have asked everyone interested here to measure their trunks for me to get a larger sample size. I sent you a PM a few days ago. As I mentioned there we can figure something out. Get back to me with what you want to do.