back of the intake manifold... most likely it is the heater core lines.

It is the easiest to pull the engine because then you have all the room you need, but it isn't required. It can be done with the engine still in the car, it is just a pain in the ass because there isn't much room to work, and it is a really awkward position. Too high up to get at it from underneath and too low to be able to rear up and around.
But it is do able.