What would make a 350 sbc motor over heat?

multiple reasons can cause overheating,usually the thermostat.But I found out it can also be The radiator being clooged,The fan clutch is faulty,or having a flex fan with out a shroud.It also depends on how it is driven. You may have the wrong size exhaust system if you or someone else have changed the engine from a 305 or smaller size. Could also be airlocked, a head gasket might be gone, water pump might be done as well. Hard to say. Depends on the circumstances and the progression of the ensuing issue. you can also bypass your heater core to make sure your heater core isn't clogged,your can also have your cooling system ckecked for leaks,and if you or someone has added alot of stop leaks to your cooling system that could also be the cause of your problems answer
the timing could be off and also you could have a reverse flow water pump (serpintine)