I’m not an expert, I consider myself always learning and improving, but I’ll take a stab at it. Other guys will offer their ideas too. Always more than one way of doing things, and no right or wrong when it comes to hobbies…
For alignment of chassis components, some guys have used Lego bricks on a flat Lego sheet to make a cradle to keep things in check. I’ve used a metal plate jig with short 90 degree walls with magnets I got from Micro-Mark, and small 90 degree clamps they offer too. I often don’t really need a jig unless parts are warped.
For longer drive shafts for stretched frames, I’ve used K&S aluminum tubing that ends of the original shaft fit snugly into, or extra shafts from the parts box, and added extra carrier bearings as needed.
For frame stretching, there are a few ways to do it, depending on what material you have to work with. In each method, it’s best to reinforce the extension with some extra plastic material over top of the joints.
if you have spare matching frame rails from another kit, you can cut the rails and insert a section from the donor rails. Usually it’s best to make the splice under the cab or sleeper area, making sure to avoid the cab or sleeper mounts, or mounting holes, so that those parts still attach as they should. Be sure to glue a strip of plastic to the inside of the rail over the joint, and also if possible, over the outside. This creates a sleeve or splice plate that reinforces the joints. You can sand the ends of the outside splice plates smooth, if they will be visible.
A lot of guys put a deck plate over the area. This looks nice and it’s great for adding strength. With this, outside splice plates are not needed.
I’ve also used sections of generic c-channel frame I purchased from Ryan Mlynek (Rhino’s model truck parts on FB or Ebay) This was a simple way to extend the frame because the older Italeri frames fit inside it, so it automatically made a nice sleeve over top. The blank c-channel frame doesn’t have any detail on it, but that is ok when it’s going to be covered by a sleeper anyway. The Rhino frame sections are the same size as Italeri’s Peterbilt kits, so that works well for those, too.
Another thing to look into is a extension kit from Auslowe. The photos belowe show a few of the options. Basically, it’s exactly what I described above, but they take the guesswork out of it. You can choose between a straight cut or 45 degree, and the depth of the frame rail you’re wanting to match. They have a few choices in length too. https://www.auslowe.com.au/shop/category/chassis-rail-extension-kit-/3643
Hope this info helps. Looking forward to seeing your building progress on the forum.