It was a less expensive alternative to the Fiberfab Valkyrie, which looked like an Avenger GT with a short rear deck and had a custom chassis with room for a mid-mounted V8 engine.

[2] Fiberfab started producing Avenger GT kits in Sunnyvale, California in Santa Clara County in 1966.

The Avenger GT's tail section was longer than the Valkyrie's so that it could cover the car's rear-mounted engine, giving it an appearance closer to that of a GT40 Mk.III.

The Avenger GT's windshield is the standard unit from a second generation (1965–69) Chevy Corvair, while the backlight is from a 1965 or 1966 Ford Mustang 2+2 Fastback.

The original Fiberfab manuals described an arrangement that used a wind-wing from a 1965 Mustang in combination with door glass from a 1966 Ford LTD and a regulator from a VW Karman Ghia.

Another option used a custom single piece glass side window supplied by Fiberfab, but the original molds for this were lost.

The GT-15 was similar in appearance to the GT-12, but included a custom chassis designed to accept suspension and drive-train components from a Chevrolet Corvair.

[7] A custom ladder chassis with outriggers was built by "Electric Vehicle Engineering Co." (EVE) of Boston, Massachusetts.