I suggest you find a used 3phase 8+ hp electric motor and use it for a converter.
Spin the 8 hp with a drill motor to start it spinning, the rest is all magic and your 5 hp will run fine.
You should be able to accomplish this for less than $100.
Duty rating has nothing to do with tank size, it is how long the motor runs as compared to how long it rests in a given time period.
Use of air tools or paint guns may use enough air the compressor will not be able to get ahead and shut off, that is a 100% duty demand.
example of duty cycle:
8 minutes on, 2 minutes off = 80% duty cycle
10 minutes on, 0 minutes off = 100% duty cycle
To all the electrical experts, lets not get into a pissing contest about phase converting and efficiency. For the large majority of users a simple converter as suggested works in 1000's of applications daily and will work fine for the OP.