Snorlax 62% (15 ATK/7 DEF/6 HP) ZenHeadbutt AND Bodyslam

Cp ? Candys ?

In general case I would say: power it up

Snorlax are really hard to come by. Its good Defensively with those moves. I would not power up if you want to use him in attacking a gym because Zen Headbutt is a really slow fast move and it takes many body slams to take a high level opponent down. You might end up running out of time, again only if you are battling 2400 and above pokemon. If battling against low level pokemon you have a chance.

This is from experience having a snorlax with body slam with lick and zen headbutt with hyper beam.

Your Lax is one of the hardest defender in Pokemon Go.
Power up without doubt, Zen / Slam Laxes are an HP tomb for attackers.
Regards