This is part of my Impala Glue Bomb re-build so I wanted to share the process I typically use in case it could help someone else who has to stretch one out quite a bit and has trouble keeping things lined up.
First thing I had to do on the Impala was to open the front wheel wells up like they should be.
Next, after I have determined where best to add the length, I get some hollow plastic square tubing that will accept a plastic rod nice and snugly.
I then super glue a couple of lengths onto a good solid point on the chassis making sure I keep them lined up with the rails. I also then make a couple of measurements and make my cut mark.
Take your razor saw and take your time to make a nice clean and straight cut.
Cut a couple of lengths of rod and super glue them into the hollow square tubing on one end of the chassis.
Now, slide those rods into the rear half of the chassis and into the hollow square tubing. You can now easily slide your lined up and stable chassis closer or farther away to match your build body.
I will usually put a piece of plastic (or dimes or whatever) under the frame just to make certain there is no sagging and then I move my chassis to whatever length I need.
In this case, I simply set the body on the car and moved the chassis out to where I wanted it. As you can see, it ended up pretty far out there!
Once it's right where I want it to be, I drop a dab of super glue onto the sliding side of the rod to lock things in place. Then I can go ahead and fill in the gap and make it all look as clean as it needs to be. That also adds all of the strength that will ever be needed in the future as far as this stretched chassis goes. Once it all starts to take shape, grind/sand off those hollow tubes we used as they were simply makeshift rails. Once in a while you can glue them into a spot where they can remain but in this case they needed to go. This build is going to have a black frame so a little squirt was applied just to see how it all blended in.
Here's the final. The frame is stretched, everything lines up perfectly, it's solid as a rock and it fits my build body like a glove!
As usual there are many ways to do this and some may be better. THis is simply the way I do them in order to keep things lined up and make it easy in the process. Hope this helps someone down the road!