The 70 mm will probably be just fine. I doubt that you would really lose that much low-end torque. Plus, when autocrossing, how often are you under say 2500-3000 rpms? Probably not that much, right?
Sounds like you're putting together a a solid combo. Keep us posted on the install and results.
Good luck,
E
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1991 5.0 LX Coupe - 40,750 miles
331 cu. in. / Tremec 3550 / BFG Drag Radials
12.22 @ 114.31 mph - w/1.89 60'
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