Go to Classic Mustang Dataplate Decoder, and put all your dataplate's info in to the correct year. It will tell you original paint colour, interior colour, what engine, trans, and rear end ratio it originally came with. Its really useful. If I put mine in it says that my car came with the C4 automatic, a 200CI engine, and 3.20:1 gearing in the rear.
You should have either a FMX or C6, the C4 was the light duty trans used for the 250 and 302 engines in 73. There were originally only 2 converts built in 73 with a C4 and 2v 351 (H code) for some reason.
The FMX is the cast iron trans while the C4 and 6 were aluminum housing.
The FMX has a cast iron center case and a removable bell housing that is usually aluminum.
The C4 is all aluminum with a removable bell housing.
The C6 is all aluminum with a non-removable bell housing.
The C4 has a nearly square pan about 8-9 inches.
The FMX has a rectangular pan with a 2 x 3 inch "kick-out in one corner.
The c6 has a large pan that is squarish with a corner "cut out".
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