1971 chevy c10 custom I6 overflows coolant after each trip

[trevarthan]

Carburated I6 250. Every time I come back from a trip the radiator overflows coolant from the overflow tube near the radiator cap. I replaced the radiator cap immediately when I noticed this happening. Still happens with the new cap. Doesn't seem to be overheating yet. Still has a quart of coolant in the fill bottle in the engine compartment.

Is this a head gasket, or something else? I blew a head gasket once on a 4 cylinder truck, years ago. It did the same thing, but it often burned white smoke too. I suppose it just depends on where the gasket blows.

I just bought the truck a month ago, so I don't know the history. Thanks!