I don't think I need to add, but just to make myself look important.....
All info above is correct including buckets info about the frame rail thickness. I looked into this when pops was wanting a K20 and wouldn't settle for a K10. All springs will interchange, and axles etc at GreaseDog and Towman said. If memory serves, the frame thickness on the K20 was 1/16 thicker, and the flanges of the rails were 1/8 in deeper. Both are made of the same lb test carbon steel also.
I also have a 85K10 I fully intend to put the 3/4 springs under and run the 3/4 ton axles.
I have a feeling the K10 frame will be better for articulaiton too, and being the square body trucks don't have a common problem of bending frames like the GMT400 trucks do, I think its a great combination. On my 85 K10, I am recabbing it with an 85 C20 cab.
If you're doing a cab swap, remember you'll need the hump out of the K10 cab. Its a higher hump to allow for the Tcase. I will have to cut out my C20 hump and weld in the K10 hump for my project.
