Though similar questions have been asked 1 and 2, neither actually answer what I need.
My house was built in 1950 and its iron waste pipes are 'exposed' in the basement. The Vent pipe is clogged, with rust and according to a professional. The professional said I should only need to replace the 90 section at the base of the vent pipe that connects it to the rest of the waste piping, but lowest price was $1,000 so I want to try to fix this myself.

He also said the vent stack needed to be secured in the attic. I have found plenty on cutting and fitting a new pipe, but nothing on securing the vent stack. Securing it makes sense since the whole stack appears to rest there at the bend and if I remove it then the whole stack comes crashing down.
How do I prepare to replace this section of the waste drain and what would I need?

