I think a lot of the conversions into full size cars are biased towards offroad so as to not stress the clutch/transmission.
The motors won't be a problem, late model bikes can do big K's no problems, it is the tranny/clutch that is affected by the extra weight a car carries.
My bike has done 50k in 4 years, no problems whatever, this is a 140hp motor that rev's to 9.5k and sits at 4.5k on the freeway all day long..
