A few things come to mind.
1 Carbs not sealed or adjusted correctly
2 Is the valve timing right?
3 Error in head gasket installation, several to choose from
4 Valves guides or rings were toast and not noticed until now, could have been heat damaged from the gasket failure
5 What I'd hope would be oil or crap in the muffler, this takes a long time to burn out
I'd run it a while and then start with a compression or leakdown test
Pull your plugs out and look at the color of the tips of them. Should be light gray in color if it is the right temp. If it is blue or green you may have coolant leak. Black is usually oil and plug will be oily too. Most plug parts books will have color chart in them or maybe check on line by sparkplug mfgs. If tip is white and plug gap is too wide then it has too much air....which will be too hot..