Al, All of your cars look great, wheels can make the car!
The Ertl 67 Corvette has smaller axles than most cars. Get some sizes of Styrene tubing to reduce the hole size of the GMP wheels and you will be ok.
As Al stated getting the old one off can be tricky. I use two flat screw drivers, one down each side of the axle and twist in opposite directions to Pop the wheel off.
Here is a Tire Wheel guide with some info. http://public.fotki.com/aztom/toys--models/die-cast-118-cars--/118-tire-and-wheel-guide/
Good luck!