I have looked at the driveshafts of quite a few usually there is a slight offset. I guess ideally straight is best but I wouldnt think there is too much to worry about. Ref the tank, usually there are 2, one each side of the rear wheels with a pipe joining them. Access to the gearbox would be compromised if you put it on top, thats the only thing I can think of and also the fact that it would raise the center of gravity on the car. I have never seen one in the front personally but apart from a long fuel run probably would be OK too.