I had my bbc w/t56 just mocked up in my oe frame using big block frame stands with the correct offset and the ATS crossmember. It did seem to fit good.
Previously I tried an older set of repop frame stands that were Listed as big block stands and that didn't work. Those stands had the driveline sitting a little farther back and outside the range of the ATS crossmember. Those frame stands also had other issues with my setup. I dont think they had any offset to them.
Anyway, dimensionally I think you may be ok. My only less than great comment about this crossmember is it seems to flex a lot. Mute point for me now since I'm moving drivetrain back which will require custom crossmember
