Unfortunately we learnt the hard way. Was going to clean the tank prior to leaving kograh bay bound for her new home at church pt. rounded Longnose pt near Scotland island and bang. 1.5k damage to a new new motor. You would think that you would clean the tank when you put a new motor in. The tank was full of algae and 20 years of crap. We used household detergent and a high pressure cleaner then left it in the sun for a few days to dry out before putting some diesel in it and swishing it around and dumping that diesel. Seem to do the trick. Your tank looks identical to mine.

prolly use metho for your final rinse as its hygroscopic and will absorb and remaining traces of water and then evaporate easily assuming you have fuel filters after your tank any ways whats in the tank shouldnt be a huge problem so long as your filters are changed regularly