Hey just a thought, but are the air cleaner elements fitted?
The reason I ask is I had a 250 X-flow motor in another car, and it would not rev past about 2500-3000 rpm.
After alot of frustration, we found it was the damn air cleaner elements. They did not have enough surface area to allow the motor to breathe at higher rpm's.
Once we removed them it revved like a champ.