I really don't think that the roof chop would be that hard. I don't think the roof will be worth much of anything after that for impact protection so it will have to be caged when it is done. The advantage of caging it is I can strip away all the now useless roof supports to make up for the cage weight. I would also be able to remove all the guts out of the doors. So I would probably end up the same weight with a cage as without and have a stronger car.
The other thing I figure would be very easy is to reshape the front end slightly to get some sort of angle on the front bumper instead of flat. I noticed the car coasts better when coasting down a hill with a curve in it so if I make the bumper and air dam sit at a 30-45 degree angle instead of flat plus maybe make it into a point so air is pushed off the sides and the top easier.
I am thinking cutting the rear narrower would be pretty easy if I don't care if the hatch works. I really don't like the idea of narrowing the rear wheels though since I like the way the car handles like it is on rails right now. I can almost double the posted speeds on the warning signs in curves with it
I know where an Opel gt is so I went and measured its windshield today and it is almost as wide as the stock metro one so there would be no advantage in using it other than it is a better looking shape. I don't know of any other cars that are small and I could use the windshield out of so narrowing the whole car seems to be down at the bottom of the list for now.
If I start narrowing the car behind the doors but not narrow the wheels I could maybe get 6-8 inches narrower on the back and the wheels would only slightly stick out. I could cover the back wheels again and it might not mess up the taper that much. I don't really know if the taper would help much though after chopping the roof down to the tail lights. The rear of the car would only be about 16 inches high. I think the front end and roof chop would be way bigger gains for a lot less work but I could be wrong.
Also venting the higher air pressure that is under the wheel skirts out the center of the back bumper might be worth something. Right now that air is just whistling through the rust holes when I go fast so I know letting that pressure out in a low pressure area might be worth some gain.
Thinking of controlling air flow how about moving the radiator to the passenger side firewall and cutting a hole in it and building a large duct that directs that air out the center back of the car. This would involve totally sealing the engine compartment and letting the air from the grille opening make its way back to the radiator then out the back of the car. It might look nuts but I bet it would help reduce frontal area. This would be easy if I cage the car since I could remove the factory floor and just replace it with a flat sheet of aluminum to make a totally smooth underbelly.