Per the 64-65 Mustang Chassis Manual, page 17, you need two ea 376428-S8 clamps. They are available from NPD in a 4 pack, 376428-S, $4.00. Be advised, unless you are really good, you need a special crimping tool as well, NPD 934-1 $16.95.
The clamp part number was available in the Library in a spreadsheet for 64-66 Mustang hardware, and in the above assembly manual. You might consider getting a set of manuals for your car's year.
Jim