My understanding is that diesels (especially the bigger ones) dominate the Autobahn. They might not be quite the fastest, but if you have to use it a lot it becomes a lot cheaper with a big diesel.
In the US, GM LSn V-8s have been great at this, although other cars now seem to have the huge overdrives as well.
Good aero is key. Aero losses will double over 70mph. Unfortunately while coefficient of drag is typically published, frontal area is not. Car companies would rather sell a CUV or SUV, while you are obviously looking for a car.
To find a car that will operate in the BSHP "island" try to find a car that has an unlimited maximum speed of ~150mph. BSHP tends to operate at 30%-50% engine load, and 100mph should be ~40% of the load needed for 150. Unfortunately most cars that can do this have limiters here or lower. [Any guesses? I remember 80s muscle cars having top speeds in the 140s and hp between 200-240, but hopefully aero has gotten better. And don't count on those hp/speed numbers being quite the same era].
Ideally you should have a gear that is at near peak torque at 100mph, or slightly lower. That should be right about the center of the BSFC island, but you don't want to be so high in revs that you wear out your engine.
CVTs aren't quite as impressive as we might hope. Que Jason Fenske:
Of course, this implies that Toyota CVTs with said gearing shouldn't have trouble at tall gearings. Still, look for tall gears that put you in the BSFC island at 100mph.
Good luck!