I have bought a chassis from a Midtec Spyder, but I came to the conclusion that it is too short for my intensions. I would like to lengthen it by app. 200 mm.

What items should I measure/take into account when doing so? I have a technical background and in terms of strength, I don't expect problems. It's only in the suspension/steering geometry that I would like to have some advice.

at which point(s) are you proposing to lengthen?

Depending where it is done, body tub say, you would need to increase prop shaft length, also posiiton of gear lever.
If engine bay, then increase length of steering column. You need to ensure your steering UJs have full movement. I don't see why there should be any changes to the steering rack i.e. geometry at the front.

I want to lengthen a mid engined car, so I have no problems with propshafts, gearchange mechanisms or whatsoever. The only thing is the overall geometry of suspension and steering. For instance the Ackerman angle; stuff like that, but is there more?