Going to post anything I find about GM Frame Thickness so we know if there is really any difference and if there is a difference, how much of a difference is there.

1987 GM Standard Frame Specifications

Key

• C equals 2 wheel drive
• K equals 4 wheel drive
• P is P forward control chassis
• S is S10 pickup
• 10903 20903 30903 are 8'0" pickups
• 20943 30943 are 4 door pickups
• 31003 is a cab chassis

In red are the recommendations if you are looking for a frame for a build and are concerned with thickness (if its a preference for thick frame)

Here the 1979 GM Standard Specifications

Notes from the gmsquarebody website

1st
*73-79 K10 & K20 frames are IDENTICAL.

2nd
*73-79 K10 & K20 frames are "slightly" thicker than 80's K10 & K20 std. frames.

3rd
*73-79 K10, K20 & K30 (single cab and crew cabs) frames are the SAME thickness. (K30's are BIGGER frames but I am just talking "Thickness")

4th
*80-87 K30 crew cab's are the thickest frames among our squares.

5th
*80-87 K20's with RPO code C6P have the same frame thickness as 80-87 crew cabs. C6P aka. HD package or I believe "camper specials".

6th
*Based on RBM (Resisting bending moment) all our frames use the same grade steal (tensile strength).

REFERENCE: https://www.gmheritagecenter.com/gm-heritage-archive/vehicle-information-kits.html

 

1991 GM Suburban Standard Frame Specifications (it says two-wheel and four-wheel drive models are the same)