What is the best way to plug the air vent in the top side of the heads. Mine leaks oil when going around curves and oil sloshes. Tube is loose, so even when I attached a rubber hose, oil still leaked out.

It's a pressed in fitting, so if it's loose, it will leak.
Typically does it at high rpm,s when there is a lot of oil in the valve covers.

You can see if you can press in the fitting better, clean it and use some JBweld to make it a tighter fit.
OR
Remove the fitting and use a threaded NPT fitting with a barbed connector.

You can tap that hole with the engine in the car, you need to remove the valve cover and keep a rag up underneath the hole to catch the shavings.

If you plug them you can blow:

1.) Front seal

2.) Rear main seal

3,) Or blow oil into the air cleaner.

Fix the vents ........ they are there to relieve blowby and windage pressure.