I would consider such a trip, absent strong tail winds and a comprehensive mapping of your airplane's actual fuel consumption at various %HP levels, highly risky.
Let me approach this from several points of view.
First, the POH data is for a new aircraft under near-ideal conditions. Your aircraft is now over 30 years old, so its engines and propellers are not as efficient as new ones, and its drag is greater than when it was new. At my most common cruising conditions (5500 ft., 2400 RPM, 21 MP) my '73 337G (basically the same aircraft as yours) uses 12% more fuel than the POH table at a speed 8% slower than on the table. So a 20% margin of conservatism relative to the table might approach actual aircraft performance. And if that's what you get, you're ditching west of the Azores.
Second, even if your aircraft shows better performance during mapping, there's no room for error here. I'm an engineer and I know a little about fuel systems (go to
www.SkymasterUS.com to see my "Fuel Supply Management" page) and you're talking of 9 hours or more in flight. Virtually any anomaly can cause you to fall short of your target -- losing your aircraft and possibly your life.
Finally, we have a member, Brian von Herzen, who has done the Canada - Europe trip numerous times in his Skymaster without ferry tanks, and he
always goes the northern route. And Brian is a meticulous planner who must have given considerable thought to this matter. That's a more compelling argument for me than all the engineering calculations.
Ernie