I was looking at an old O Scale News magazine with an Overland Models ad for the SD70Mac that lists the minimum radius as 35". I remember seeing another Overland ad for the SD90Mac that lists the minimum radius as 38". Overlands website also listed 40"minimum radius for their SD40,45T-2's that they canceled several years ago.
I have several Overland SD75m's and a SD70Mac that will run fine on Lionel O72(36"radius) fastrack without a problem. I can't run anything coupled to them shorter than a 50' boxcar because of the severe overhang of the loco's couplers. The driveline doesn't make bad clicking noises on O72, that would suggest that the u-joints are being pivoted to their maximum limits creating binding, but I think they're very close. I've also tested my loco's on 40,45 and 49.5 radius Atlas sectional track and while they run very well on any of those radiuses, I think a 49.5" radius would be my minimum if my space available prohibited me from larger radiuses.
I have a friend that runs his SD70Ace on Atlas 49.5" radius and it runs very well without any problems, and it looks decent on this size radius,and seems to be operating comfortably within it's design limitations, I prefer a 56" radius for loco's in this length range. I don't have and SD70Ace as of yet, but I'm tinkering on my layout with 58" and 60" radius to satisfy my personal aesthetic preferences.
I think that a safe minimum for loco's of this length range(C44-9,SD70Mac,75m)from a design perspective, that wouldn't cause premature wear and/or failure of driveline components, would be nothing less than a 45" radius.