I went to a 700r4 in 1989. No one in the area that I could find could build a 2004r that would stay together for me back then. I am back to a 2004r in mine. Lots of knowledge out there to build a stout 2004r in the last 20 yrs.
That said I would go 2004r probably into the high 10s . After that I doubt it is a street car that gets driven. I would do a th400. Nothing wrong with a 4l80e. Just more complex. Computer, adapter, driveshaft, torque converter, moving the crossmember.
To me if you are planning to drive it and run in the nines an lsx swap starts to look pretty good. At that point I either unload the car and start with a regular g body, mustang or f body and go fast or lower my goals. Cutting up a turbo regal for safety stuff just isn't in what i would do.
Are you near a builder of 2004r that is mentioned on here? Plenty of them can get you what you need or even ship it to you.