If you are using a recovery bottle, are you also using a cap designed for recovery systems (mid '70s and up design)? FGFs did not use recovery systems but it is a common addition.
The fill line on the radiator is about 3" below the neck. If you fill higher, it will spurt out on shutdown. This is not the same as boiling over. A properly positioned recovery bottle and cap should be able to manage the flow.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
