Stay on IBM POWER 6 and accept that you will be on an unsupported version of IBM i in April 2018 and continue without IBM hardware maintenance after the 31st of March 2019.
Stay on IBM POWER 6 and accept that you will be on an unsupported version of IBM i in April 2018 and take out third-party maintenance after the 31st of March 2018
Upgrade from IBM POWER 6 to a IBM POWER 9 server that does support IBM i 7.3/4 and will have IBM hardware maintenance support beyond March 2019.