Center of stock big-a$$ coupler to back of cab is 53.5'
The overhang of the cab roof pushes it back to 56'
If you ignore the massive front coupler, the distance from the tip of the cowcatcher to the back of the roof overhang is 54'.
I scratchbuilt the turntable on Effett Yard 8" specifically to fit my 2-10-2:
Will swap out the Hulk Fist couplers for MT905's soon!
(And install a Tsunami sound decoder in tender as well).
Hope this helps.