I have given this a LOT of thought. I have a 72 LWB Cheyenne C10 Super. I already have a short bed, not installed. My frame has been re-straightened and it measures out "X" method. It has trailing arm rear suspension. I'd like some opinions on my frame shortening method, positive or negative.
I plan on sliding the suspension cross-member forward 12". This leaves 23" of straight frame behind the member, take 12" straight cut out of the center of that area and that leaves me with 11" I can box in with the CPP frame stiffeners. I can drill some holes to bolt the stiffener channels inside the rails (or clamp) to help align when I weld the outside of the frame rail, then flip them the other way to box it in later after welding the inside and grinding welds flush. I could fish plate before boxing the stiffener plates, using the metal from the section I cut out to fashion those.
At this point the welded part of the frame is only stressed for towing and the weight of the bed and load. If you cut/weld in FRONT of the cross member, then you have accel/decel/bed and load/and towing all stressing on that area. I will use really good bolts to re-install the suspension cross-member and make sure they are big enough to fit snugly into the holes to prevent walking and re-use the lower frame straps to reinforce the lower frame with 4 bolts on each side. Maybe tack weld it in a some spots?
Before I make my cut (to minimize frame drift), I slide the suspension cross-member out of the way and measure 2 new REAR holes EXACTLY 12" forward off the original REAR 2 holes on the bottom of the frame. Using the rear holes, bolt the cross member in and drill through the front two holes through the lower frame rail using an angle drill downward, then drill from the bottom upward through the upper rail. Bolt the 2 front upper to rail, then remove the lower rear bolts and drill through the rear upper frame rail and install those bolts. Install the two lower bolts each side with the stiffener strap that came off the bottom, drill the extra outboard holes through it through bottom of frame and bolt it in. Now it fits right having only made the critical 12" measurement off the rear two holes. THAT is your critical measurement that puts the wheels where they need to be.
I believe I can easily do all this with the cab still on, I can slide the cross-member under the cab when installing my new cab/core support mounts, leaving the rear cab mount brackets where they are.
Some people like to weld up cross and X braces in case the frame wants to twist or jump around after the cut, making alignment difficult. My long frame has been bent and re-straightened so I'm sure I will have to do that.
Please critique this method if you feel it needs it, I can only benefit from that (the reason for the post). Feel free to ask any specific questions about this method. Here's a link to the frame stiffeners I want to use.
http://www.classicperform.com/Store/...ck/6372RFS.htm