OK, A '50 Chevy with a new stock 250 and auto transmission. I was thinking of the older vac advances. You have and HEI. Is it a stock GM or aftermarket? It doesn't really matter. Can you check if the previous owner was having this issue? With all you have done we know a lot of what it is not. If you are sure the timing is right, there are no air pockets, and the pump is moving coolant it makes me think about restricted flow or too high temp thermostat. I don't think a fan shroud is an issue here since the car didn't have one. The first easiest & cheapest thing would be a 185 degree or so thermostat. If that doesn't help some then the radiator is likely the problem.
Does it actually boil over before or after you shut it off? Do you have a recovery/overflow tank? It is not unusual for the temp to climb after shutdown especially with high temp thermostats. A 195 degree stat opens at that temp. The engine is going to run at near 200. This is an emission thing not necessarily a good for your engine thing. This is where the pressure rating of the radiator cap comes into play. When the pressure in the cooling system exceeds the rating of the cap it pushes against the spring and allows the expanded coolant to get out hopefully into the overflow tank. It can be drawn back in as needed with no loss of coolant or mess on the ground. This is a more modern concept than the '50 Chevy used.
You have a new engine and parts with new paint and grease and sealants. It is going the give off "burning" smells for a while. The hoses and other parts will feel very hot that is why we don't take 200 degree baths. All of this heat stuff is relative.
I'd like to see some pictures of this cool little ride.
