top speed

Hi all. I have an 850 Mk 2 Commando engine with Maney stage 2 head and a belt drive, running in a classic kneeler sidecar racing outfit. Just about to install a 6 speed gearbox (Hemmings), and need to work out the best gearing (sprockets) to get highest speed at 6,500 to 7,000rpm. Don't want to...