Yes the 450 SEL has several inches in the rear pass section. Find a pic of one on the web and you can see it is longer in the rear door.
I think you can replace the key switch with only pulling the cluster rather than the whole dash.
I have the factory shop manual for the 116 and while there are some differences in the cars, I think it is very do able.
There is a different vacuum plumbing schematic, but the vacuum elements themself
AFAIK are the same.
Don't know about the drive shaft for sure but probably no more than 100 bucks. Look for a truck repair shop that does drive shaft work. They will be able to fix you up.
I would only do it if I could find a really clean SEL and have a donor car which I guess you already have?