426 Hemi

Hiya guys
Long time since i've been on, busy at work and all the boreing stuff but know its time for the engine swap,but i have a couple question you guys might be able to help me with.First one why do some people put spacers between the chassis and the crossmember what bennifit would you gain from it also i will be mounting the engine with a motor plate what do you think my chancers are of getting these headers in and retaining the torsion bars they came off a 63 fury and are tti's.
Cheers john