I'm thinking 4+ stops, maybe 5 as a safety margin. Red doesn't ring true to me with this sort of film. My thinking comes from the effect of using R72 filters in IR. I think your best action would be to bracket a few frames and run notes on each exposure, based on the standard test methodology of exposing IR film.

Mind you, the colours as they present (in Fllickr) are wild, off-the-planet — enough to make Velvia blush!