Holley's come with hollow copper floats, hollow plastic floats and plastic floats which are filled with honey comb plastic foam similar to the datsun hitachi carbs. Your need the filled plastic float so it cannot crush under boost.
You need mechanical secondaries
You need to make sure your throttle shaft bushes are good.
There is a company ( forget who) that sells rubber gaskets for Holley's, but with the way the holleys bolt together, they arent required unless really high pressures are used. All the mating surfaces of these carbies are thick enough to prevent bending and releasing pressure under boost.

EDIT: Stress the solid float is a major requirement. I have seen what was left of a hitachi copper float after it was squashed and popped from boost.