1975 Kawasaki F9 Bighorn cylinder temperature hot

[wade12819]

Greetings,
I have a 1975 F9. I blew the piston and have since replaced the piston, had the cylinder re-sleeved back to stock, re-installed the oil injection pump that had been removed and replaced the left side crankcase seal along with the rotary valve cover and seal. It starts easy and is running well except it breaks up at 6,000 RPM; I didn't attempt to push it past that. Of course this is a concern but I have a bigger, perhaps related concern.
My bigger concern is the temperature of the cylinder. After warm up and an easy 3 mile ride the cylinder temperature was 280 degrees F. I waited 10 minutes then rode another two miles at 50 MPH, (4,000 RPM) and the cylinder temperature was 307 degrees F. The ambient temperature was 90 degrees F.
I understand the running temperature for liquid cooled bikes is around 215 F but I don't know what temperature to expect with this bike.
Will someone please tell me what the "normal" cylinder operating temperature is for this bike? Is 307+ degrees at the cylinder cause for concern?
Many thanks,
Steve W