All 63-66 T-Birds came with "Extra-Heavy Duty" shocks as original equipment.
There were no exceptions! Most dealers sell cheap "Heavy-Duty" shocks with cheap prices.
If you buy the cheap priced "Heavy Duty" shocks your not putting on the original equipment. The car is much too heavy for "Heavy Duty" shocks.
Always use the -Extra-Heavy Duty" original Motorcraft shocks!