How low can you go?

I'm starting out with 12" 150 lb rear QA1 springs on Strange coil overs on my triangulated 4 link (aka Madart). Car is a somewhat more portly '74 Space Duster. Was going to step up to .920 torsion bars to match but now I'm conflicted about converting to Badart's new coil over kit in the front. Anyone have some input on the front spring rate (many spec. 400 lb in front). What does this equate to in handling = Road race, autox, street/strip?
I'm setting the car up for HP street and beginner road racing but it will see the strip and hopefully some road trippin. Thanks, Rapom.