How do you think a 3.8 exhaust would be hampered by the stock LUV torsion system?
Seems to me it would fit without much issues at all.
Leave the torsions in and have a REALLY GOOD muffler shop route your exhaust properly; should be no problem!
Your coilover desires should be resolved if you do a simple search here on this site just by typing in CoilOver in the search selection.
There is one done here that is a simple and direct approach to this.
I will pass on the 5-lug bolt pattern for someone else to chime in with their suggestions!

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72 L.U.V.
427 SBC-700R4
SE 9-3/8 4P TracLoc, 6-lug rear discs,
custom rear leafs, custom made aluminum radiator,
Mustang II front suspension
LUV perf parts manufacturing
My Devotion to Karl :
http://www.youtube.com/watch?v=EmXc4F78 ... re=related