Tom,
It all depends on how close the barrels fit. I have fitted a couple myself, that were real close. And I've had a couple fitted by others, that weren't so close.
As to the forearm, it to can go either way. If your fitting a barrel that has it's own forearm, I would try to use that forearm myself.
I can highly recommend Dewey Vicknair for the work if needed.
http://www.vicknairrestorations.com/index.html