mgriff, I can't help you with a specific measurement as I put a 700R4 in mine.
However I think you will be very accurate if you take a different approach.
I assume that you have the transmission. Mount it to the engine and with the engine in place on its mounts you will have a fixed point as to where your cross member will go.
Now just mount the cross member to the transmission and attach it to the frame at that point.