Hello!
It's an interesting question. Shortening is easy, isn't it? Now for stretching I'd see two options for you. One is building new rails - and while it sounds compicated, I thing you're better off here than just building the inserts and glueing them somewhere. First, you can get parts that are finer than AMT parts, and the cleanup is also much easier. If you use 1mm styrene sheet (0,04'') the rails are more than strong enough, you could also try something thinner for the flanges - 0,75mm (0,03'') or 0,5mm (0,02'').
My second option for you is to buy an Italeri Freightliner kit - it has nice, long frame for you and you could also use other cool parts from the kit, like the front axle with workable steering and it's got air ride! - detail on the Italeri kit is generally finer than in AMT kits, just a pity that they don't do the cabover.
You can take a look at the Italeri parts here:
https://www.mojehobby.pl/products/Freightliner-FLC.html
Any way you go - good luck with your projects and be sure to upload some pictures! Have a nice day
Paweł