I would suggest lowering the top for two reasons. 1, reducing the frontal area will go a long way to reducing your CDA. 2, the lower the height of the car, the shorter your boat tail needs to be, the easier it is to make a larger percentage of a "complete" 100% boat tail "fit" onto your car.
Lengthening the wheel base is essential for a better boat tail. For my car a 80% boat tail would add between 6.5 and 7 feet. Then i have to deal with issues of ground clearance for hills etc. I f i could move the rear wheels back i wouldn't have that problem.
Look into Dave Clouds "Dolphin" for what he did to a second gen Metro if you haven't already.
http://ecomodder.com/forum/showthrea...hin-13142.html