I'd go with a Type III cooling system and a side-draft dual-choke Weber carb or EFI throttle body; under a flat bed.
I liked the 5-speed conversion I got in the 90s. With 185-60s, it shifted up into 5th at 85mph. But driving around town, there are two gears to choose from instead of one. Unfortunately, the 2nd gear syncro problems have persisted through the conversion and a rebuild. If it goes back into something, I'll just live with the 3-2 downshift.
You started the thread with a brown Beetle/Squareback hybrid. Maybe it's worth pointing out that the BGW fiberglass panel conversion has ideal aerodynamics, as I determined with the experiment I showed in Permalink #22:
It smoothly continues the taper of the body. An added 3ft boat tail would be perfection.
