I've bult and run several size dredge's in the past 2 , 3 , 4 and was in the process of constructing a 5 inch subbie when my time of dredging was ended due to my health . I never got into the bigger size dredge's like your asking about BUT why not call Keene Eng. and ask one of these guy's about these big size nozzles / venturi's ! I did just that and Mark Keene answered all of my question's and made a few pointer's for me to look into ! Big help to me ! There was at one time (on the net ) a guy selling directions to build your venturi's from detailed plan's . I don't know if they are still there but it may be worth a lookie - see if they are still AVA. ! Goggle " Bonaro" he used to post here a lot and is a real nice guy!!!