I would highly recommend actually leveling tank if you're interested in playing it on lvl 70 content. It is the fastest class to level due to the high demand in the duty finder queues providing you nearly instant queues on pretty much any dungeon/raid you select.

Additionally, if you dont take the time to learn the class from the ground up, I would highly suspect that you may be put in situations in lvl 60-70 content where you may not know the ins and outs of the role you're in and other players may not have the nicest opinions about it in the duty finder. If you're playing with a group of pre-made friends to learn it, then by all means go for it, but I would provide that bit of caution of you're dropping into a fresh level 60 tank acquired by boost potions and trying to enter content in the duty finder.