Yes, you must purchase the EJ25D timing kit since you have pretty much adopted that setup when you converted to DOHC heads. Make sure you use the 97-99 EJ25D heads as their chambers are well enclosed in the EJ22T bores.
Also, the passenger side head must be tapped for the turbo & since you're converting from single to DOHC, you may as well get an 11mm STi oil pump (to compensate for the increased room in the heads) & a new turbo water pump as well as new tensioner & timing belt pulleys.
Set the timing as you would an EJ25D & it will start right up. The crank pulley, crank, & cam sensors will be the same & your ECU will run those heads. You WILL have a decreased compression ratio though but that will allow you more room for boost.