I used the calculator on the jenvey sight. You need 300mm from head to inlet side of throttle body. That will give you top 3rd of 7krpm. 350mm gives you mid to top end, 400 gives mid, 450 bottom to mid, 500 bottom. With the space in the bay you will probably end up 400 max. I went with 330mm but I did plan on fitting trumpets to bring that closer to 400. Never got round to it. 330 didn't bring the power band low enough for me so I'd definitely try to go as long as you can fit.