I think I would live with it for a few cold/hot cycles. You may be topping it off each time and it doesn't want to be all the way to the neck. I had a flathead that would do that and as it turned out 3/4 of the top tank was all it wanted. As the coolant expands it has to go somewhere. See if after a few cycles it stops spitting.

"Old trucks are not a matter of life and death, they're more important than that."

1968 Chevy C-20 3/4-Ton Short Bed Dually