Drew - Did you by chance remove/reinstall those rear running board brackets from the frame during your build? According to the 3100 frame diagram in the 1947-54 Chevy Truck FAM (Sect 2, Page 2.00) the rear bracket should be mounted so it angles forward at the outer tip (see pic). The frame checking measurement for the distance between the center line of the center support bracket to running board holes to the center of the rear bracket inner mounting hole for the running board should be 27.436" + or - 0.062" (approximately 27-7/16" to 27-1/2').
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~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)