Are you looking to be able to remove the roof and doors and leave the roll cage and screen on or will the roof be perminantly on the car?
If your planing the former then it will never be water tight, little short of chauking round all the joints it will leak to some degree. If you go for the later then just bond the roof section to the body work.
There was a car based on vauxhall parts with the engine mounted at the rear called the Cyclone made by Carcraft, it had a square nose and a tessarosa style back end. It had a removeable roof section and removeable gt40 type doors.
