440 with slant 6 torsion bars

Agree that what you need to do depends on the primary intended use of the car. For drag racing, the /6 bars will work well enough. For street use, a higher spring rate is desirable for the TBs. If it is going to be a street car, the /6 bars will get you through a state inspection, but won't be a lot of fun to drive hard.

Jacking up the /6 TBs might be able to bring the RB up. The extra weight is more than the /6 TBs and OEM shocks are designed to control on the street. Likewise, the /6 leafs will wind up like crazy with a 440 unloading power through them.