A friend had an aluminium wing made for his Dodge Dart, it doesn't have a duck bill on the ttailing edge.
Just wondering how this may effect the cars handling etc

Not exactly sure what you are describing, a "Gurney flap", which normally is just a small flat strip added to upper railing edge to significantly effect DF of a wing, or maybe you mean wing Camber, which in a cross section line maybe described as banana shaped. Camber is the centerline between the top and bottom aero surfaces. The Wing chord is a straight line between leading tip and trailing tip. Its a rather complicated subject IMO, testing is always the final process.

The Gurney flap

Look on youtube for videos by Tim McAmis. He does a nice video on rear wings. It's all about the weight, power level and speed of the vehicle. Yes, the duckbill/gurney flap creates a significant amount of downforce. The height is determined by the looseness of the car on the top end.

When I added the wing to the back of my car I
had major issues.. at 135 mph the back of the
car would lift off the ground.. I didnt know
this even with video.. I talked to the wind
tunnel guys and they showed me smoke footage
of a production car like mine.. I could see
where the smoke would drop down past the back
of the car.. they explained that the back of my
car was in a negative pressure zone and caused
lift.. once I put the wing on it was day and night
difference.. the car was controllable and picked
up a few MPH and drove great.. I tried 2 different
versions.. I didnt like the first but it worked..
then I added the longer flatter one which I have
on it now.. I put the spill plates on it for 2
reasons.. so the air wouldnt slide off the ends
and give the wing more strength