IMO ride height should be the first thing to determine. It dictates your pinion angle, front ladder bar placement, and shock mount placement.
Regardless, set the car up like DD said, 25-37% of the shock should be in the body to determine were to place the x-bar, hopefully you have spring that are weighted correctly. When that is done and you have the car at ride height rotate the diff to get your pinion angle, there is alot of info on this forum already just serach "pinion angle"
I would really try to use the leftover x-member to go between your frame conectors and rockers, notch it to go over your conectors and but up against your x-mermber, it will really help tie everthing together.
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