Pretty much if not all the frames will say don't heat/drill etc. on them. The top and bottom part of the "C" is a definite no no as far as drilling heating. The vertical part of the frame rail as long as it is done within reason is not really an issue for adding aditional holes. (provided you dont make it swiss cheese) I am in the process right now of having my Kenworth T2000 frame stretched. I went back and forth between doing it myself and having it done. I decided to have it done but learned alot along the way. I have a higher than normal tensile strength frame on mine as it is only 1/4" thick. More common I believe is 3/8" and not as high of strength. For my stretch I am adding 8' infront of the drives, and need about 7 feet out the back (planning to be 42 or 43 long, leaving the 6' condo sleeper and adding a 26' box). I ended up finding an identical frame with crossmembers as my current truck and had it cut under the cab. I am having it turned around backwards and butting it up to the rear of my exising frame. The frames will be butt welded and have a fish plate on the outside and a 2nd piece of "C" frame added to the inside 4' infront and behind the welded seam. This allows me to not alter the existing frame and saves on a bunch of labor to not have to re line up and drill all the holes for the suspension components for both axles.(atleast on my truck the axle mounts and air bags are all symetrical so they can be mounted forwards or backwards and it will still line up) By going this route I have the same frame rail that was built with the truck and only 1 seam that is supported both by being bolted fishplate on the outside through the existing frame and new inner frame as well as welded the original frame. Probably overkill but I am also pulling a 25k lb stacker trailer.
If you are only going with an 18 or 22' box you can probably get away with just dropping the front drive axle, and adding a few feet on the rear? Just depends on how much tail swing you want or don't want.
Good luck,
Steve
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