It may not fit your concept so well, but said wealthy patron strikes me as the kind of fellow who might take an old warship and convert it into a yacht. (Like Onassis did with the
Christina.) Take a surplus
Rudderow-class or a Castle-class, put new engines in, etc.... The Rudderow is 300 feet long, which gives you plenty of room but might be a bit big.
One note: one of the restricting factors on Mississippi navigation is the height of the ship; Huey "The Kingfish" Long
had a bridge built on the north end of Baton Rouge that has a clearance of only 113'. The legend is that this was to keep ocean-going traffic in Louisiana, not headed further upstream.
Also: When I think '60s boats, I think hydrofoils.