A picture is worth a 1,000 words:
Pictures and desciptions from www.Mustangsunlimited.com
View attachment 485585
FRAME RAIL,REAR 65/68 RH (COMPLETE)
This is a complete rear frame rail, from the torque box back to the rear of the car.
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FRAME RAIL FRONT LH 2-PIECES 65/70
The frame section that travels from the firewall to the radiator support. Both the flat Outer Section & the Inner Box, once welded together it will form a box.
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View attachment 485589
TORQUE BOX,REAR 65/68 LH
View attachment 485591
TORQUE BOX,REAR 65/68 RH
This is the front 14" of the rear frame rail and the torque box. It attaches under the rear seat and includes the section where the front of the leaf spring attaches.
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View attachment 485592
TORQUE BOX FRONT LH 65/70 COUPE/FASTBACK
View attachment 485593
TORQUE BOX FRONT RH 65/70 COUPE/FASTBACK
This is the 3-sided section of the frame commonly called the torque box, it is visible to the rear of each front wheel.
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Finally, the best investment you can make in your car is the "Shop Manual":