Put them along the back edge of the left side, right side and top of the canopy. They "should" be placed every 4 inches on center. But you would end up having a lot of them on the car. So I bet you could double that distance without any major decrease in the effect of the VG's. Getting the air to come off the back of that big boxy *** end in a smooth fashion should help.
Strangely enough, the biggest improvement I have noticed is not so much gas mileage...but rear end stability. My Mustang was generating alot of turbulance and some aerodynamic lift before the VG, because of all the dirty/sloppy/turbulent air flow. The VG's helped alot by making the car "slip: through the air in a more clean fashion and not the rear wheels/*** end of the car are much more stable and it gets more stable with more speed. Instead of less stable with speed.
__________________