Pictures would be the best way for anyone on here to help you with your specific situation. No matter what you have there is something that will work. Whether it is galvanized or brass does matter, one is going to become troublesome over time and that is galvanized. Watts makes compression products that tight seals to O.D's of piping, meaning you can turn an upright for the trap and use a compression ferrule (rubber) to make the connection to the strainer. Take pictures.