You shim both of them.

You need to make sure that the mesh is centered, which is why you shim the pinion forwards and backwards. (Like a clutchbell)

And you also need to shim the outdrives to control how close the mesh is. There should be a tiny amount of play. If there is no play there will be plenty of friction and heat, which will wear the pinion/bevel gears out quickly.

Its all done by feel, much like the article rights. Too much slop and it will wear out quickly. To little free play and it will also wear out quickly. Its hard to write it down here, it best if you ask someone in person so they can show you the amount of play there should be between the pinion and bevel gears.

Kinda like learning how to shim a clutchbell, or setting the mesh between clutchbell and spurgear.