As Eris states, you should have plenty of power and if you choose a high blade tip speed you will be golden.
The correct engine rpm though to achieve the 540 rpm PTO speed is not the maximum RPM of the engine, check your owners manual for the rated rpm for the desired 540 rpm pto speed.
As a sidebar, there is nothing wrong from a mechanical standpoint at running the engine for a prolonged period at its rated rpm. In fact, it is better than running the engine slower and potential lugging it. In fact, I would hazard to guess that if you did add a aftermarket gearbox and ran the drivetrain slower than it was designed to be operated while still producing the horsepower and torque required it would shorten the gear train (including the engine part of the drivetrain) life.
The L3800 engine is rated at 2700 and the PTO(540) is 2580RPM. So is the "rated" refering to max rpm or where the engine makes peak hp? If it is max rpm, I just don't like the idea or running the engine at 95% of its rated rpm.
So far all have said that 38 hp (30 PTO) will handle a 72" finishing deck with no issues. That being said, I would like to run my engine at about 2100 (75% duty) for mowing which at best guess would give me 22 hp at the PTO running an overdrive gear. A B2920 has 21 PTO hp and will still run a 72" mower just fine. I plan on using a finish mower only for the yard and will not be mowing tall grass with it so I would think that the tractor would never have any bogging issues. I know I am just being overly anal about the engine rpm, but I just wnat some feedback because if I have the necessary power, why not invest a few $ now and save on the engine and fuel consumption over 30+ years might make this mod worth it.