How do you change the oil in a ski centurion boat with a 351 ford motor?

Easiest way is with an electric pump through the dipstick tube. This works best with warm oil. Shur-Flo pumpbuckets are available online or at most marine stores. Most centurions had an extension tube to a fitting on the oil pan. You run that tube through the bilge drain (with boat on a trailer), and then take the plug out of the extension tube.