long bed bed to frame bolt pattern

[ray_mcavoy]

Originally Posted by v30crewcab

every one I've ever had the bed bolts go thru the frame at the front leaf spring mount. 1/2, 3/4, or 1 ton. but they were all 81 and newer.

Evidently, there is a bit more year-to-year variation than I thought. I just checked a few examples in Dad's junkyard and some reference photos I had saved of well-kept originals from various online ads and found the following:

1973 GMC C15 parts truck with no bed, but holes in frame at #1 & #3 (no holes @ #2)

1977 GMC K15 (pics from Rust Free Classics) using holes #1 & #2
1977 Chevy K10 (pics from ClassicCars.com) using holes #1 & #2

Dad's 1977 GMC K15 with E81 wood floor using #1 & #2
1977 GMC K15 w/steel floor using #1 & #2
*I took a close look at the spring hanger reinforcements on these and found that they would not interfere with bed mounting bolts at the #3 location as I had previously assumed. In fact, the reinforcements have holes punched in them where bed mounting bolts could pass through. However, the frames on both of the 2 above listed K15 trucks do not have corresponding holes in their frames at location #3.

1977 GMC C15 (pics from an old Barrett Jackson listing) using #1 & #2
1979 Chevy C10 using #1 & #2
*These two examples contradict what I posted earlier, being that they are both 1/2 ton 2WD, but using #2 instead of #3

1984 Chevy C10 using #1 & #3
1987 Chevy C10 using #1 & #3

I also have an 8' bed with a steel floor that came off an unknown truck (I acquired it as a homemade trailer on something other than a GM frame). I believe it's off a 78 because it has the newer style tailgate hardware, a round gas filler hole, and black accent side trim. I don't know what it came off in terms of GVW or 2 vs 4WD, but it has mounting holes at the #1 & #2 locations.

Based on this info, it looks like GM may have switched back & forth over the years, with there being a time in the mid 70's to early 80's where they all used #1 & #2.