i watched someone do it on a show where they installed a Mustang II front suspension in an old Nova
they used a bare block, bolted up what they already had for mounts, then measured out the tubing and the braces
i'd suggest using a jack from the bottom to help stabilize it, and hooking up the tranny/crossmember so you have a guaranteed good point of reference (unless you're going to change that as well)
trying to measure while it's hanging... that just doesn't sound like a good idea to me, too easy to have it move somewhere