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The 1966 Comet Cyclone GT put Mercury's 335-hp 390 into a new midsize body. It came as the 63H two-door hardtop or 76H convertible, both with bucket seats. The GT name belongs to the complete package, not a separate engine page; Mercury's AMA sheet also uses GTA for the Sport Shift automatic version.
The 390 FE used a four-barrel carburetor, 10.5:1 compression, premium fuel, dual exhaust, and a 335 gross-hp rating at 4,800 rpm with 427 lb-ft at 3,200 rpm. A fully synchronized three-speed manual was standard. Buyers could choose the four-speed manual or the GT-only Sport Shift Merc-O-Matic automatic, with all three selectors mounted in the console.
A 116-inch wheelbase, short-long-arm front suspension with coil springs and a stabilizer bar, rear leaf springs, and a semi-floating rear axle defined the chassis. Mercury's October 1965 AMA sheet specifies self-adjusting 10-inch drum brakes; the final Comet shop manual limits its front-disc statement to eight-cylinder Mustangs. Because later summaries and individual restored Cyclones conflict with those factory records, this page does not present front discs as standard or as a verified factory GT inclusion without a final order record for the car.
Published totals list 13,812 GT hardtops and 2,158 GT convertibles, for 15,970 cars. Those totals do not split GT from GTA, transmission, axle, paint, interior, brake, or pace-car presentation. Hood scoops, stripes, badges, a 390, or a later disc-brake conversion cannot prove a factory GT by themselves; body style, warranty data, drivetrain, dates, and car-specific records must agree.