Spur Gear design - how to remove interference

Hey all, I'm new in the forum. I signed up just to make this question, because after Reading 4 mechanical books i couldn't find the answer for this (there was none topic for interference in those books - can give name if you want ).

I'm trying to design a multi-stage garbox but i can't get over the first set of gers beause of interference.

I know that you ca remove interference by two ways : center distance and reducing and increasing addendum. My question is, for the first option, what do you do? For the second option, how does it modify the calcs for strength and how do i know how much i need to modify the addendum?

The specs are: pinion 13 teeth, gear 33 teeth, 72 pitch (if you need more info, let me know)

I'm not being able to edit the post so i'll edit it here.

The 2 options that i was asking is the modification of center distance and profile shifting. Could you give me a hand?

Grateful