I would like to use the A-518 overdrive tranny in my B-body with my 440. I have read the articles on what to do and how its been done, but here is my question, Can the internal parts of a 518 be put inside a big block tranny case or is it totally different? I've heard the 518 described as a 727 with an overdrive added. Is there more to it? I'm not fond of using an adapter plate or cutting and swapping bellhousings from one to the other.

the OD case is specific to the A518, so you can't swap anything over from the OD part of the unit to a standard 727 case. the ONLY option[s] you have is the ultra bell, or a swap adapter plate.
from the OD part forward, however, the internal parts are interchangeable, but the case has internal passages where the tailshaft/OD unit bolts up, preventing just bolting up the OD to the 727 case, and getting it to work.
myself, i'm going for an ultra bell. [as soon as i score an A518 core]

Thank you, Moparx. I will probably do the Ultra Bell also.

Lots of guys have done it here. I did two years ago with advice from this forum. Still running good behind a stroked 512.