Thanks for your replies and advice.
I should have made it more clear that I need more horsepower for uphill speed. The topspeed of the dukw is fine with me, but to reach the top of a hill in 2nd or 3th speed with about 15 to 20 mls/hr is not frustrating for me but more to the modern cars behind me that can't overtake me on the secondairy roads.
We had a amphibious meeting this summer in Norway and sometimes there were about 30 passenger cars and trucks behind me waiting for me to go out of their way, so they could move on with normal speed.
Perhaps it's better to stay on the water, there are no hills
