Your bow will work fine. Millions of animals have been taken over the millenia without heavy poundage bows. Keep your shots inside of 20 yards, and make sure they are clean shots with good angles to the kill zone.
Either arrow can kill an Elk, the heavier one should penetrate better for you. The 515 grain arrow put in the boiler room will also do the job. If you exceed the state minimum poundage requirements for you hunting area, get out there, hunt hard and only take shots you know you can make. Don't change up bows in the middle of a season. That is a guarantee to make a bad shot.