Okay, here's some more info for those who have questions.
FWIW, the MTH Premier Big Boy measures 35” and works with O-54 curves, so I’m not sure why the concern that the 34” turntable is too large. Now I don't know if that's the engine in question, but it addresses the reason for the 34" turntable with the O-54 curves. And remember, this is not my design, just my conversion from paper to SCARM. 
If you check the Korber Models website, they have templates for their 3-stall roundhouse and add-on stalls. They even make a spacial add-on kit for the Big Boy (and other long engines?). I didn’t find these before because I didn’t see the links in the product descriptions or the data has been added since I looked. Anyway, they show various configurations with the roundhouse using different position points and different distances from the turntable. They show how the whisker tracks then fit inside the stalls with the most desirable distance appearing to be 16.20” from the turntable so the engines are dead center. The exact measurements of the roundhouse are 5" for front doors, 9.48: for back walls, 30" for standard side walls and 36" for Big Boy walls. It turns out I got close, but didn't do so well on the width or the placement. It's much closer than 16.20", so it has to be moved further from the turntable. Since I now know they make a separate add-on for the Big Boy, I can reconfigure the roundhouse to have only 1 long stall and 4 shorter stalls to see how I can best fit it without encroaching on the aisle.
