1965 c10 build

[rusty matheny]

Building a 1965 c10 short bed.
Frame seems to be out of square 1/2 inch. Local frame shop says this isn't an issue on these trucks but, I am concerned .

Fairly decent hp and torque going back in ( 420 hp/torqe ) , tremec 5sd, lowered ( not bagged ) , sway bars etc.
Concerns are not as much about lining up body panels as fit and finish can be adjusted for, my concern is it pulling straight and steering correctly- really need advice from someone well experienced with these frames, proper checking, tolerances and where to look for/find issue.

Can not locate single area where frame is out, ALL cross measurements are pretty close to 1/2 inch out over every section of frame ( three x measurement areas) - it almost seems as if frame was "out" from factory.
Cannot find any apparent bends or creases in C channels.
Im just stuck , no frame shops in my area want to deal with bare frame and all seem to think 1/2 out on square with 50 year old truck is ok.
Please advise/help/direct.

Thanks ahead of time.