Help! 67 gmc k20 71 k20 cab frame mount difference

[AdamH.]

I'm not sure how to post pics. I'm going to use the 68 k frame and put a 71 body on it . (The 71 has rust issues. The frame cab supports are in same location but the hole punched in them is different. The 71 hole is punched farther back towards rear if cab. The 68 is punched basically center. I believe this and core support bracket locations will effect body panel alignment and maybe the box clearance. I'm wondering if in 71 it was wrong at the factory. Inputs please. Thanks!