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!

[57taskforce]

I’ve never heard of the cab mounts being in different locations. Interesting. How far off are they? The core support mounts are definitely different but should be in the same physical location.

[AdamH.]

The cab support holes are off 1/4 to 3/8 back.(center) The core support are in a slightly different location also. The hole size is obviously smaller on the 68 core support. I thought this stuff was the same to. Maybe just the difference in plant manufacturer location and year. Or gmc to chevy. I'm fortunate to have both frames side by side. Also someone slotted the bed mounts back some on the 71.Thanks. Anyone have a pic of k 71 frame?

[AdamH.]

Pics

[special-K]

I've never seen or heard of this before. I guess it with take some mock assembly to see what's what and how things will fit. It can be remedied if there is a fitment issue, but this sure is an oddity

[AdamH.]

Could someone provide me with center to center measurements from core support to cab support holes. 1969-1972 k frame. Thanks

[landarts]

I can provide measurement today if you provide picture of exactly what you need.

[kwmech]

If the measurement is off I would elongate the hole and leave everything loose until all is assembled and aligned then tighten it down. That (I think) is the only way it would look correct. Be interesting to see what's off. Both sides like that?

[AdamH.]

Yes both sides. Tonight I'll take a new pic

[gmc684x4]

Rad support hole size should be the only difference 68 has a smaller hole then the 69 72 trucks

Prothane had a disclaimer in there cab mount kits about this to open the hole up wider or cut the collar off the body mounts

Cab mounts themselves should be in the same actual place as the cab is the same 67 72 c10 c20 c30 k10 k20
K30 would have been a napco or diy thing

Put your cab and front clip on and test fit to the frame

[AdamH.]

This is the measurement I'm wanting. From center of radiator support hole to front cab hole. The 68 is about 50 5/16. The 71 is about 51 1/8. I know the angle of tape is a factor but I had it on same angle on both frames. Could someone measure there's like this and tell me what they have. Thanks

[57taskforce]

That is interesting, my ‘72 K20 has the holes centered like your 68. This is a couple old pictures of mine. Unfortunately I didn’t take any pictures of the front mounts specifically. As it sits my front mount bolts are more or less centered in the frame mounts.

[Palf70Step]

I know the core support mounts are different for 67 /68 that they are for 69-72. I wonder if that is what might be making the measurement different? If you are going to use the 72 on 68 frame you may need to swap the core support brackets also. Otherwise they should be the same.