Welding pressure vessels is not something you want to experiment with. You have to know what you are doing and go through plenty of destructive and non-destructive testing to become certified it that field. The tuition for the school of hard knocks for that can literally be an arm and a leg or much worse.
Build your pump and motor mount then attach to the tank with some steel bands, instead. Or buy a tank with mountings on it. Tanks are hydro tested after they are all welded up. So even if you have the skill, it's not worth it to do the testing.