The Corsa top part comes off easy, it's just held by friction.
Then the "Tube from EPAS body". It's also a friction fit but it didn't want to come off.
I welded some nuts on it and used a pulley puller to get it out.
It's not even necessary to take it out, you can carefully cut it to length with an angle grinder.
I have cut the TR7 Column here:
The TR7 steering shaft here:
The tube was cut here:
Corsa steering shaft here
Then welded the TR7 shaft on the Corsa shaft.
The top of the TR7 steering column was welded on the piece of tube that's sticking out.
That's it!
Regarding the alignment, I just put the Corsa housing in the vice.
I spot-welded the shafts and turned them around.
I have used a Dial indicator DTI but that was too sensitive.
The slightest misalignment you see immediately and I got them quite straight.
Hope that helps!