A 3.91 rear gear with the Passon 5 speed would be a pretty good compromise. That combo would have a 10.5 overall first gear ratio which is maybe a tad steep but it should work. With the 0.70 OD ratio the top speed at 6000 rpm would be 165 mph. Not sure the engine can push the car that fast but it might. Tim has had the car to 155 mph and he said it had more in it so maybe it will go 165.

The carb is only a 650 so if a person really wants big numbers rather than drivability then there is another 20-30 hp available from putting a bigger carb on it.

Only problem I see is that the driveshaft wasn't designed for that rpm. With the 0.7 OD the shaft speed will be 8600 rpm at 6000 engine speed. I don't think the aluminum shaft can handle it so the car will probably need a carbon fiber shaft.