Regardless of engine, ALL of these cars are under suspended compared to modern cars. Biggest favor you can do yourself is the following:
1--Obviously, inspect and or rebuild the suspension as necessary. If it has not been touched, I'd replace all bushings and ball joints
2--Consider some sort of disc brake upgrade. You should do this BEFORE rebuilding the front end, because you either need to make the decision to keep your old 4" wheel bolt pattern, or convert to the 73/ later parts and the later 4.5" bolt pattern
3--Get stiffer torsion bars and INSTALL a front sway bar. Replace the shocks
4--Using modern radial tires, do NOT use factory alignment specs. Use the chart on this page (the skosh chart down the page):
http://www.allpar.com/history/mopar/...alignment.html
NOTICE that camber is the OPPOSITE of factory, IE original specs for bias tires have the tire OUT at the top, more modern radial specs have the tire IN at the top, or negative camber
5--Rear suspension has many options and sources of rear springs. In my case, though, I'd junked out a newer Duster and a Demon both, so I took one set of spring packs apart, as well as my 67 originals. I then stacked the LONGEST spring leafs out of each pack, I believe ending up with 1 more than factory, and the bottom one is one leaf longer than the original. This gave me a little lift and the stiffness I want without being harsh. All it cost me, other than the rear axle swap I was doing at the time, is a set of U bolts and new "spring clips" which you can buy at any spring and brake outfit, or order from many places on the www