This doesn't really need a thread all it's own, but for the sake of knowledge and organization here we go:
First, the 122 seat base. I've removed the lasagna noodles for a better view:

The rail is bolted down through the top into captured nuts. The dimensions of these bolts are 310mm fore-aft and 345mm side-side. There are two sets of holes fore-aft so you can adjust the seat further back than the sliders will allow.


Under the 240 seat, with the slider rails removed, we see the framework:

The holes are the same 310mm fore-aft but are 365mm side-side. As with the 122 seat base, there are two sets for fore-aft adjustment, as well as many holes up/down for tilt and height.


But fortunately, these frame pieces unbolt:

And with some 10mm spacers and longer M8 bolts, the frames can be brought inwards the required 20mm. Here I've used the ubiquitous B21 cam gear washers, which are about 6mm thick. Being that the holes in the frames are oval the extra few mm doesn't really get in the way of things:

And Walla!

The slider release mechanism fouls on the 240 seat frame so some adjustment (grinding) may be required, but this seat went together such that the adjuster did still work in this position.

So you can see, installing 240 seats in your 122 is pretty much painless and doesn't take a lot of effort. You could leave the rails in the car, but bolting the frames to them afterwards might be a struggle. Might as well pull them, it's only three bolts! This process for a 140 series car is probably somewhere between this and impossibly easy. I didn't take photos of the seats in the car because these seats aren't actually going IN this car, I just knocked out this job in about 20 minutes just for giggles.