'65 Frame: Help Request

[SCOTI]

Tonight I was working on a 6X swb bare frame & was doing the prep work for squaring it on my semi-frame table. As I was documenting the diagonal frame numbers for future reference, I noticed the measurements were off.

Here's the weird part...... When I measured the 'front' diagonally (from a hole on the rail adjacent to the front cab mount to a hole on the opposite rail adjacent to the core support), my numbers were good (square).

When I measured the 'rear' section of frame diagonally (from the farthest-forward front bed mount hole on one rail to a hole almost @ the end of the opposite rail), I'm off by 7/8". I re-measured using different holes toward the rear (half the distance vs. the initial measurement) & was consistently off again.

I cut the center c.member out tonight (intact) & didn't notice anything binding & then installed an ECE c.member w/no issues.The frames I've messed w/so far have all been within a 1/16" so this is new territory for me.

Would any quality frame place (collision center) have the needed info to verify everything is good to go?
Because of the age, where can I get reference material (dimensions) specific to these old frames?
Does anyone else think it's weird that the front numbers are good while the rear numbers are off (yet the frame doesn't 'look' like it's tweaked)? I put a 48" level against each frame rail (between the cab mount area) & the frame is flat there.

Finally (for the locals) . ..... Where can I go here in Dallas/Garland area to get this frame checked so I'm not wasting my time?