Here are the spec's for the 1965 mercury comet, 289 V8:
For 1965, the Comet Cyclone was powered by the 289-cu.in. V-8 with 225hp, the same engine that was put into Mustangs. And for more power, there was a special high-performance package, better known as "Hi-Po," available with the Cyclone, and it shared the 271hp V-8 which, when installed in a Mustang, was known as the "K" code car. In the Comet, the engine code is "A." When equipped with the Hi-Po package, a 1965 Cyclone came standard with: 289 four-barrel high-performance engine, four-speed Toploader transmission, nine-inch differential, performance suspension, quick-ratio steering and 8,000-rpm Rotunda tachometer. However, the following items were extra-cost mandatory options with the 271hp engine package: four-speed transmission, heavy-duty front suspension, low-profile rayon tires and 42-amp alternator, according to 1965 sales information.
Its a 9-inch differential at best. Try this link for more data:
1965 Mercury Comet Cyclone