While a 1500 and 2500 frame may look the same and be constructed with the same tensile strength steel, they have a different section modulus.
In layman's terms: real 2500s are "stronger" than 1500s.
The simple calculation looks like this:
RBM = SM X YS
RBM = resistance to bending moment
SM = section modulus
YS = yeild strength
It is much more complicated than that, but the simple equation is good enough for this discussion.
The half and half 1500/2500 trucks I have no idea on. Someone would have to dig up a gm engineer to get the specs they used. I suspect it is the same as a 1500 though since the "LD" 2500 uses a smaller front differential like a 1500. The larger 2500 diff won't fit a 1500 without alterations.
Its been a while since I looked, but I think I recall the 2500 and 3500 had the same SM and YS values. I have no desire to go dig it up again though, that stuff is a fairly closely guarded secret by most OEMS. They don't like to let it out because their competition would use it to thier advantage when designing thier frames.
Gm used to list SM in thier dealer info packages, but it disappeared somewhere around the mid 90's when the "my truck can haul more than yours " wars started. Its a useless number for salesmen anyways, all they could say was it was "bigger" and buyers usually had no clue either. Add to that, SM is a useless number on its own. Its how it effects the calculation that matters. Same as the YS number, useless on its own. Its how it effects the calculation that matters.
Often, you'll see tensile strength noted in add hype. Thats because they're big numbers and they sound really impressive. "New frame made with two hydroformed 45,000 psi tensile strength steel frame rails! Highest in any new 20xx pickup truck!"
Sounds impressive doesn't it? But if the SM is lower on the higher tensile strength steel, the frame could actually be "weaker". A frame with lower tensile strength (is: YS) might actually be "stronger" depending on SM.
Ah, marketing. Give 'em half the truth and get it out the door.....
Frame design is a science all in itself. Most people don't realize that because they never tax a frame to its limits in day to day use. The OEM's design it that way on purpose for liability reasons as much as anything else.
But those that tow or haul hard, long and heavy would be wise to either not mess with the oem stuff or do a lot of studying before sparking up the welder and sawsall. Chopping, welding and bolting stuff together to get it to fit doesn't mean you'll get the results you expected, there's much more to it than that.....
Same reasons are why a snow plow can crack the boxed front frame on these babies. Yer messin' with the oem design and it doesn't like it....
Personally, if the op wants a dually short box I would say go buy one. They're available, not very expensive (relatively), it will be a full 1 ton truck and he'll be able to confidently load that sucker down without fear of failure or anything "weird" happening.
Or getting pulled over with a heavy load and being questioned about what the gvwr is...cause it will still just be what is on the door placard as far as officials are concerned.
It was my dads truck and it is too dam long and a short bed dually is sweet.
