NFPA 85 Boiler and Combustion Systems Hazards Code for large boilers,
is controlling above 12.5 million Btu/hr, while
ASME csd-1-controls-safety-devices-automatically-fired-boilers
is controlling below 12.5 million Btu/hr
There some exclusions in both.
373 boiler horsepower ~= 12.5 million Btu/hr
So your 300 HP boiler is covered by ASME CD-1, depending on your jurisdictions boiler safety code.