Good point on the contour. It's the simple things you sometimes don't consider.
I always go mid length on a 16" but this is a wildcat round, a .338 Spectre.
After speaking with the guy at SBR, who kind of took over for TeppoJutsu on the Spectre, he has told me that I should really try the pistol length first, so I'll give it a shot before considering moving the gas port.