BH,
Are you looking to position the Tranny/TC support?� If so, I found it easier and more accurate to measure from the center of the rear leaf spring mount, forward, to the Tranny/TC cross member mounting holes.� They are at: 20 1/2" and 23" to center.
If I misunderstand what you are trying to measure, post back and I run out and do it again.
Tim