Upgrade for '70 Swinger 340 rear

This is just what I would do.....
I'd keep the 3.23 suregrip intact, and redo the 3.23 open with 3.91s and a clutch suregrip from Dr Diff. And get some good axles with big bolt pattern from him, while I was at it.
Agree with above, ss springs and good shocks should be plenty, even with drag radials, and lowering blocks if you want. I have ss springs on my car, and it sits just fine for me, not even remotely stinkbug. But then, I got the 3400lb springs for my 3400 lb car.