Argess If you are looking for long term life of the motor, this is the golden rule I live by and shift by. Anything higher is just wasteing motor. This is why it is important to match all the parts to worh togeather. Carb, intake manifold, heads, compression, exhaust manifolds and most important on a cobra, sidepipes
Have to be able to get the gas out of the car. If your motor is stock, 5,200 rpms will be about the limit with max hp and torque being there. 428 motor is a torque motor, not a HP motor. If your motor puts out 400ft of torque. That's a strong running motor and very streetable. If you are looking for a little more power, the work to the heads, intake, sidepipes would be needed. This includes a carb, intake and camshaft change. I raced a stroker 452 motor with 9.2 compression for 8 years. I could run with cars that had more hp car all day. The drive train and getting the most out of the car has alot to do with this. Cobra like about 550 HP and 600ft of torque. This is about the edge of the safe limit and drivibility of a cobra. Yes we have a couple of guys on this forum running 700+HP motor. They are track cars, not saying that you can't drive them on the street but they where not built for street. If you FE motor is stock bottom end, the 2 things a would suggest is that you run a 7+ quart
oil pan and HV
oil pump with an 80# spring in it. This is for high spirit horse play. MSD with a 5,700 chip in it also. If the HP craze bits you, build another motor and keep this one as a spare backup just in case.
Rick L.