I like a 12t clutch and 60t-70t rear sprocket with a stock to stage 2 212 engine when running a newer 6" wheel bike.
Your rims are 6" not the tire. 6 inch OD tires with a 212 is a recipe for a wreck. I have a bike with 4 inch rims and approximately 9.5 inch OD and I don't ride it. You get the death wobble going and it's over.
My drag bike with a 6" rear and 10" front wheel is dead steady at 70+mph.
My CT200U is running 6" wheels and tops out right around 60mph. Steady as can be too.
My DB30 with the stock wheels will do maybe 50 and it's steady too. The brake is the sketchy part on that bike.
I used to have a bike with 5" wheels, that thing was scary at anything over 25mph. That's the last pic, started out as a Cat and got modified. A buddy still owns it and I want it back, he won't sell it.