Some guys are true geniuses. They can visualize every little thing and plan six steps ahead.
Me, I'd have to have a fully dressed motor/trans to mock up. I'd have to stare at each part that unbolts, to make sure the bolts can be reached and the part can be removed. I'd also have to study the angles, exhaust clearance, where the fluids drain, etc. Guess I'm slow...
You can go with a big pipe that will fit into the bearing saddles to get an idea of the driveline angles. The old FED builders did this to align the drivelines. http://www.condercustom.com/ATF/purgatory/chassisbook.html