Most people who have the 300's and most people who've built the 300 inline-6 agree that 2000 rpm is where peak torque is made, and 3000 rpm is right around where peak horsepower is made. The power curve drops off significantly past 4000 rpm, and if you go past 4500 you risk over-revving the motor, floating valves, and other problems.

The reason a 300 can't rev high is it's long stroke. These motors have a 4'' bore, with a 3.98'' stroke, and using 6 pistons, that makes the 3.98'' stroke a big stroke. If it was a 302 with a 4 inch bore and a 3 inch stroke with 8 pistons, you could go up safely to around 4800-5000 rpm because the piston strokes are an inch shorter. Past that, I'd want the motor balanced and better flowing heads.

The key to remember here, is that the 300 doesn't rev high safely because it doesn't have too. All of it's power is on the down low, which is where you should keep it when having fun.

Peace.