About a "few hundred" Fitch Sprint Corvairs were built by John Fitch & Co.
It is believed that only 2 of the Fitch Firebirds were built, the surviving OHC-6 and the now-lost 400 CI car. These FFs were built in '67 with both performance & handling modifications. The subsequent Pontiac in-house Firebird Sprint Turismo engineering package built upon Fitch's efforts and led to the '69 Trans Am package. One source says he sold Firebird modifications packages for $1000, but I have no verification of that.
Earlier, Fitch had built a sports car circa '49-50, simply called the Fitch. Using the smaller Ford flathead V-8, a fiat 1000 chassis and a highly modified Crosley Hot Shot body, his 1520-lb roadster did 0-60 in 6.3 seconds!
Fitch built a "Toronado Phantom", tho I have absolutely no further info as to what that encompassed.
He also built a single prototype of the unique Fitch Phoenix in '66, another sports performance car intended for limited volume production, but when he learned of the impending federal safety requirements, he abandoned plans to build any more. It was based on the Corvair & used a modified 'Vair flat 6. The one '66 is alive & well tho:
More pics at: http://www.coolcatcorp.com/limerock/fitch.html
Fitch is an interesting & innovative guy, as of '99 (at age 82) was working on liquid-cooled brakes whose benefit was greatly reduced unsprung weight.