Time for a Long block?

First thing is to do some serious diagnostics. Especially since these girls are "so old" you could be "anywhere" on this

Engine wear in general, rings, valves

Valve seals leaking

Carburetor problems

Ignition problems including but not limited to a stuck advance mechanism, and some amount of wear, generally, in the distributor.

If you don't have, stop by MyMopar where you can download (some) free factory service manuals. By the way, some of those came from the guys here on FABO

http://www.mymopar.com/index.php?pid=31

what I would do, "just so you know" where you are

Run a compression check and cylinder leakdown test.

Read the engine section (9) of the manual, re: the cam and cam drive. Determine the wear on the timing chain, and whether it's slipped. The procedure is in there

Set the valve lash if they are adjustable.

Examine plugs for carbon, fouling, oil, etc, and wear

Test the plug wires for continuity, examine the cap and rotor for grease, moisture, dirt, carbon tracking

Inspect the distributor. Read section 8 of the manual, electrical, theres a section on there about distributors. IN THE rear of that chapter is "specifications" which detail vacuum and mechanical advance in the distributor. Set and check the timing, and attempt to determine if the advance is working correctly.

By the way much of what I detailed above is simply "a major tune up"

Once you get done with compression tests,valve lash, and ignition, NOW you can pay attention to carburetion / intake

Along the way are OTHER "gotchas" EG is the heat riser butterfly stuck / rusted, which can become a partial blockage in the exhaust, and speaking of which INSPECT the exhaust. It's not unheard of that someone "drags" the car over a curb, log, or "errant sand bar" and mashes part of the tail pipe shut!!!