it's not done quite like that, unbolt one side support the engine, cab front spring hanger and pull the rail out from the back of the truck
I would use K&W truck repair in Searcy Ar. 501 742 5085 talk to Kenneth Jackson I have seen them stretch and shorten they have plenty of references and do quality work or they won't do it. They will build and balance your drive line in house as well. Not to mention from time they start till done is amazing some times. they do those kinda things so much they are just set up for it.
A local shop here(Stockton, CA) quoted 8K out of door(double frame) for a Pete 387 with final length at 45' (overall and what ever wheel base I want). This is for motorhome conversation.
You would have to talk to Kenneth but I have seen several done there like you are talking about just no idea of price.
I worked at a Freight liner shop for many years. I did many shortenings, stretches and rail replacements in my 20 Plus year at the dealership. Using factory pre-drilled rails--Blocking the truck, eng., trans., radiator, and sleeper if the truck had one securely. Remove the fuel tanks, battery box, 5th wheel and anything else like drive axle fenders and mudflats. Secure the steer axle and proceed to remove the bolts or huckbolts. Using a small forklift gently pull the rail out onto roller stands. Reverse the the steps to install the new rail--repeat this for the other side.This would take two men between 72 & 96 man hours depending upon the adons on the rails etc. If the rails are blank you will add 20 minutes per hole to be drilled.This pushes the turnaround time very high at todays labor rate plus the cat of drill bits and reamers. Hell the cost of new bolts washers and nuts is scary.
