WOWSERS that is clean! Excuse me while I roll my tongue back up off the floor.

If truly a 'frame off' I would imagine they used new rear springs. Something does not compute. Maybe they cheaped out and left the old springs there? Maybe the shocks are shot? Does it bounce around? I agree with Bigchief- don't go crazy with HD springs. You want compliance with the road. How does the front feel? I had front discs in my 1968 hard top. WAY better than drums. Now I have 4-wheel disc and that is what you want. Lots of aftermarket kits to make that happen.