There's a bit more than just changing a few hard parts that goes into a well built transmission. Knowing what parts to use and which parts to modify can make or break a build. As long as you know what you are doing, go for it.
Even guys like Art Carr can't figure out how to run a lock-up converter with them. There are some shops out there that specialize in building these specific transmissions, but not many. That speaks volumes to me.