when it goes on the ferry across to France
A privately owned bowser full of 100LL? On a ferry????? I wonder.
I'd have thought any hassle (why?) refuelling in france would pale into insignificance compared to the the grief you'd get trying to take our own bowser onto their airfields and refuelling yoursef. Jeez - they'd go nuts and rightly so too! You wouldn't dream of doing that here, would you?
And what's special about spring? If you're avoiding wind you want stable high presssure and that isn't springtime, is it? Winter or late summer/early autumn I'd have thought. All you need is just one of those wonderful gin clear flat calm perfect skiing days to get you over the mountains and you're into the valleys on the N or W side of the Alps and that's the trickiest bit over. But if you haven't had mountain training I'd leave the Alps alone in such a machine even in those conditions. But then by far the worst turbulence I have ever encountered was on the coast near Marseille and it dam near overwhelmed us in a Gazelle. Utterly terrifying.
I don't know about computerised planning either, I'd be happier doing the work myself and getting a proper feel for the route and everything around it. In a 60mph machine wind is a serious influence. A 2 hr leg can be anything between 80 and 160 miles with just a moderate 20Kt breeze aloft, diversions, bolt holes and crit points are going to be needed, plus a healthy awareness of the effects of changing groundspeed on your CP. Planning by machine will only distance you from the reality of all this.