Myself, I went for 45 km/h officially (unoffially, on a flat road, I could reach 53 km/h). While turning, for safety reasons, I limited myself to far lower speeds (25 km/h).
Designing a car suspension system for reasonably high speed seems hard, I have never tried, instead choosing the robust and crude solutions to get a reasonable assurance of strength.
Motorcycles seem easier. Especially since most of factory-made motorcycles use a sprocket and chain - a very flexible system for dropping in other power sources. I imagine that with enough know-how to get through type certification, a lot of combustion bikes could become e-bikes with excellent riding characteristics. :)
If the approximate weight of an 18650 cell is 50 grams, 8000 of them would indeed be 400 kg. Probably more, since they must be packaged somehow. But the pack can be split into modules which a one person can haul around somehow.