#1 - Changing from 14" rims with bias ply tires to 15"+ rims with radial tires.
#2 - Larger torsion bars. I used the old Mopar 0.990", but 1" is good too.
#3 - Front Disc brakes if it has front drums.
#4 - Modern stereo system and speakers
#5 - A good fitting / quality dual exhaust system
#6 - If 2-bbl carb, converting to 4-bbl and intake
#7 - Changing from point ignition to electronic
#8 - A higher output alternator with wiring mods to handle the extra current.
After the above, depends on what your doing with the car.
Automatic trans car: Trans cooler, TF-2 Shift kit, higher stall converter
A good cooling system if the car run hot
Fuel system with bypass return to prevent vapor lock
All the above assumes Engine, and other systems are in good condition to start with.
If it has 8-3/4" or Dana 60 rear axle, make sure to check, lube and adjust the axle bearings. It's a bummer when your rear tires pass you going down the highway (ask how I know.)
More common is to upgrade items that need replaced anyway.
If you need new fuel and/or brake lines, you can upgrade to stainless steel and run 3/8" fuel line with return. 1/4 return was standard on some cars, but 5/16" or even 3/8" return would be good if planing to run EFI, or high volume / electric fuel pump.
If your thinking of running an electric fuel pump, and your fuel tank needs replacing too, consider the fuel tanks with the built-in fuel pumps, the in-tank pumps run quieter and cooler in the fuel tank.
If the trans needs rebuild, good time to do shift kit, and other changes. Don't forget U-Joints, and a driveshaft safety loop is a good idea too.
There are too many engine mods to list, but check the timing chain for slack. If you find a car this old that has the stock timing chain set, it likely is worn out by now?
