For most applications, the stock setup will work. The first thing to upgrade would be the radiator. A nice four core will make all the difference in the world. And Be Cool even makes a awesome aluminum radiator that will drop in. And while it costs about $500, a new four core will run a little over $200. If your motor is close to stock, or even mildly modified, then that four core will work great. Note: the AMC 401 is notorious for overheating when the block is overbored too much. If you have a bored 401, then the Be Cool might be reasonable. Also available from Flowcooler is their aluminum water pump. It costs around $79.
A stock AMC water cooling system is 10% overdriven by design, so that at low RPM's, the motor should have no problem staying cool as long as the components are in good working order. Modifications only need to be made when spending time in the higher rpm range, and with high compression.