Try under exposing the film by 1 stop with the green filter in place and over develop the film by two stops. This will have the effect of increasing the density of the negative in the highlights which will increase the contrast and help produce brighter whites in the final print especially if you use grade 4 or even 5. The reason for underexposing is to compensate in the shadows for the extra development. You are likely to see some increase in grain but this will help produce the grainy effect of IR film.