I wouldn't do it. The El's will not last long and your liable to damage a grid resistors. From what I understand a bias mod should get you there. I'm sure someone on this board has done it. Any modders in Italy?
Just out of curiosity, why EL34's? They will make it a little tighter and give it more of a mid hump, but at the end of the day it's still a Rectifier. I preferred EL's but it wasn't night and day IMO. You have to ask yourself if the mod is worth the money to you.
Thanks for representing the Rectifier line in Italia!