Recently I came across a post on the Vintage Thunderbird club's site

 where the post was inquiring about installing

 either air or spring assist shocks to help raise the rear end of his 1965 Thunderbird.

 

We offer 4 options for accomplishing this: 

The first 3 involve load assisting shocks

 and the fourth is replacing the leaf springs.

1964, 65, 66, Thunderbird, T-Bird. 

Spring assisted shocks are the most durable option:

XHD SPRING ASSIST

PXL1115 is recommended for the most boost per pair and the bigger piston size.

 These shocks provide 1,100 lbs of additional carrying capacity per pair

 and have a 1 3/8" internal piston (50% bigger than HD). 

SPRING ASSIST

PL1115 spring assist shocks provide approximately 850 lbs

 of additional carrying capacity and have a 1 3/16" internal piston.

Air Shocks let you to vary the amount of boost, up to 1,100 lbs per pair

 of additional carrying capacity per pair and have a 1 3/16" internal piston. Always keep air
shocks inflated to at least 25 psi to prevent damage to the shock boots.

Leaf Springs - Made in the USA to stock specs, raised, or lowered.

We can custom make coil and leaf springs, to stock specs

 or we can modify them to raise, lower, soften or stiffen the ride quality.

 (http://www.vintagethunderbirdclub.net/phpBB3/viewtopic.php?t=3785)