David (the norwegian the_van_man is referring to) is pretty damn crafty with a welder if you have a look at his other car.
Yes it can be done but it would be a hell of a lot of work.
I think you'd be better off getting a whole doner ke70 sedan, cut the rear floor out of both, and then weld the sedan floor into the wagon. That way the geometry for everything is correct. However you'd also want to bear in mind things like the fuel tank location in both vehicles.
Yes coils are better than leaves, but there's still not really anything wrong with leaves if they're set up well for the application. Just think of some successful race cars of a by-gone era like and RX3 which ran leaves. Hell, ZR1 Corvettes can do sub 8 minutes laps of "the 'Ring" and they still run leaves. :thumbsup:
Personally I think for the money and effort required you would just be better off setting up the leaves well...... or buy a new coil sprung car.