hmmmm... What kind of tires and what air pressure? Did you scale the car? Curious what the rear weight is compared to the coil over (I assume it's coilover suspension) spring rate... Does the rear of the car lift, sag, or stay neutral when you launch? 4 bar rear suspension???

I would say it's a suspension issue... A Willy's coupe, even if it had a full belly pan, should never get into an aero condition enough to unload the rear..... A wing IMO wouldn't be a fix, just a band aid....

A bit more information about the car's construction would help........