I also agree with upgrading the original frame.

One hting to keep in mind on these is that the VIN is located in 2 places. One tag screwed to the inside glove box door (that some states don't recognize as it's only held on with 4 screws and also stamped in the right front frame rail. Change the frame and you loose the VIN (someting to think about BEFORE you toss the old frame).

An older setup that works really well on these is the bent torsion bar suspension from the late 70's- mid 80s RWD Dioplomats and New Yorkers (the down sized ones.

I did one of these years ago with a 302, C4 and 9" and loved it.