Just remember that if it looks unroadworthy it's likely to get pulled over by the cops as they'll think its a hazard to other road users.
Also, it's got to be comfortable.
This would seem to count out extreme rat rods with mail slot windows, no floor and bomber seats.
There is another definition of sleeper. That of a vehicle in which one would sleep. i.e. a camper.
Where's that '65 Elky kit...