The Superbird was the Plymouth response to Dodge's 1969 Daytona. Same idea: nose cone, tall rear wing, slipstream-cleaned body — but on a 1970 Road Runner shell rather than the prior Charger 500. Plymouth's specific motivation was to bring Richard Petty back from Ford, where he had defected for 1969; the wing-car was part of the deal.
Production: 1,920 cars, well over the 1,500-car NASCAR minimum for 1970 (the rules tightened from 500 in 1969 to half of a manufacturer's annual production for that body, which Plymouth interpreted as 1 per dealer for the eligible dealer count). The breakdown by engine: 1,084 with the 440 Super Commando 4-bbl, 716 with the V-code 440 Six-Barrel, and 135 with the 426 Hemi. Lehto's research is the canonical reference for the splits.
Designer — the underlying body was John Herlitz's-era Plymouth Studio work on the 1968 B-body (shared with the Belvedere / Road Runner line). The Superbird-specific aero — nose cone, wing, flush rear window, vinyl top to hide the leading-edge weld of the rear-window plug — was engineering work led by John Pointer's team that had developed the Daytona, applied to the Plymouth body with Plymouth-specific tooling. NASCAR rule changes for 1971 made the wing cars uncompetitive (the 5-litre engine cap killed the Hemi's advantage, then the 1971 banning of the wing cars closed the chapter); both the Daytona and the Superbird were 1969 + 1970 only.