I am wanting to buy a fuel pump for a '95 Olds Achieva.?

You have to drop gas tank in order to replace. How can I assure myself of buying correct pump with out taking off the old one first? Are the pumps compatible with each other? For example, will an AC Delco, work in the place of an Airtex brand?

tank replacement

That is correct. the car needs to be raised to drop the tank. You would need tools ( a brass flat punch and hammer). You will have to disconnect 3 lines and electrical lines. the fuel pump should interchange with the rest of the N bodies. You can look for a fuel pump for the Pontiac Grand Am as well for the same year.