We ferried our Seneca I non-stop from Libreville to Maun two year ago - 11 hours flight time. We had two ferry tanks in the cabin and full mains and landed in Maun with two hours of fuel remaining.
We can lend you a ferry tank if you want to PM us. Jimmy The Gong borrowed the other one but he may not have used it yet and will be returning it.
PM if we can help!