I mounted my 3 point on the 66 like stangnet did.
I did not take pictures of mine but there are some here:
Front & Rear Shoulder Belts
The down side is the upper point is below your shoulder and can compress your back in a wreck. I would rather have that than be impaled on the steering wheel or face planting the dash.
An alterantive for the 65-66 is to mount it on the roof but there is not a strong mounting point there like in the 67 and later Mustangs. You can add metal to form a anchor point but you will need to remove or pull back your headliner and weld in metal. I saw someone who had done it but can find it in my safed links.
Good Luck and Be safe
Ron
Christophers 66 Mustang Restoration and Modification
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A code 66 coupe
289 4v, Roller Rockers, Electric fan
Performer & 650 Edelbrock
MSD ignition
Tri Y into 2.25 Shelby side exhaust
T5 conversion & Hydraulic clutch
CSRP Discs