Wow. That really did go quickly. Forgive my lack of car-building knowledge, but next you basically insert an additional length of squared steel tubing between the front and rear cuts, and weld it back together?
How did you decide where to cut?
How will you ensure that everything lines up perfectly true again? I assume that's extremely important...
This has been a dream build of mine since childhood. It's really fascinating to watch it unfold, and at this speed. It's moving quickly enough to be entertaining - thanks for sharing it with us!
Hey Westies! Yes, clearly this has been a dream of mine too

I CLEARLY remember the first time I saw the trailer for the first Batman movie, and thinking how cool the car looked, even though you only saw glimpses of it.
I think the key to a project like this is pacing. Steady pacing. I don't plan on moving quickly, but I hope to move steadily.
As for your questions, yes, you have the basic idea right. Once the car is stripped down, the frame is basically two long bars running the length between the wheels. You cut those, add in a length of box steel, then weld it back in place.
The sort answer to your question about keeping the alignment true is this: i'm going to hire someone to do it. I've read up on it, and it basicaly comes down to plotting some measuring points on the frame, measuring up, down, and diagonally. Then you run the math on how those measurements will change with the stretch. You then clamp the new steel in place, make sure your measurements are right, then do the welds. There are techniques you need to use when welding so that the heat doesn't warp the entire frame. You basically do tiny welds, and jump to the other side of the car to do a tiny weld, then jump again, etc.
BUT, I'd much rather just hire a guy who has done car stretches before, and leave it in his capable hands. I will do all the other welding myself, like building out the c0ckpit bucket, the supports for the shell, etc. But the primary, load bearing welds will be professionally done.
I'll be sure to film it when the dude comes in to do the welding, so stay tuned.