Gluing Post Base To Concrete
I would do the following:

support the roof - and jack it up a 1/2” extra to enable the new one to slide in.

drill and cement or epoxy the post support ( stainless is good) into place, these are usually adjustable so getting the roof level is possible.

measure the required length of post - from the support to the roof and the 1/2” can be accounted for in the adjustment.

fit the post - the bottom support may be a “cup” which might need one side reducing or it may only be a U, which means the post end can slide in easily.

adjust the post height while removing the jacking supports.


May be worth treating the post ends and painting before fitting though.