CAUTION!!! This post is full of
OPINIONS. 
@BlueBird, you're probably not going to find any catalog shocks made by the upper-end guys for the '64-'66 Thunderbird. IIRC, and I could be wrong, the rear shocks are unique. At first we tried the usual suspects like
Auto Zone, Advanced and O'reillys and every rear shock they gave us was too short. Their inventory systems are complete bull pucky for classics. In the end, we purchased from either Macs, before the takeover, or Pat Wilson. It was definitely a T-bird specific vendor and I believe the shocks were just black with no labels of manufacturer. We put those shocks in before I learned about Bilstein and I must say that we are satisfied with them. I know some of shock companies tell you how to measure for their products. The other way would be to find a
part # from major manufacturer like Monroe and then call up Bilstein or Koni and see if they can cross reference. My personal opinion is to stay away from KYB, the cheaper Gabriels, or any shock cheaper than $20 per wheel; most definitely, do not use Monroe Load Handlers or air shocks. Air shocks work fine when sized properly and with good springs but unless you have a level ride setup they aren't worth the cost or trouble when compared to a good shock. I am not referencing air bags here, just air shocks. 64RM's air bag setup is a different animal and is really cool.
@Dj-Dan I think Bilstein is a German company. IMO, the Bilstein shock has great ride quality and longevity but at a cost, figure around $80 a wheel. On a scale of 1-10, I would put them 1 point higher than Koni, 3 points higher than a generic Monroe or Rancho and like 4 points higher than a KYB. For me, KYB is an ok performance shock with bad ride quality.
I should mention that I've used Bilstein on our daily driven trucks, suvs, and moderate performance cars from the 60's and 70's BUT not on our T-bird. It is my opinion that they offer the best compromise of ride quality and performance. Some will say that they are over priced and over kill for a daily drive. Every one has an opinion and many are valid. Asking what's the best shock is kinda like asking about colors, everyone sees a different shade of the same color. I'm not well off financially but I am comfortable and have no kids so I can afford to blow and extra $40 a shock. If price is a concern then I totally recommend Monroes or whatever the better T-bird vendors are selling for a daily driver. On a side note, my brother just purchased a redesigned front suspension kit that replaces the king pins with ball joints for his 51 Ford and surprisingly the kit came with the same Bilstein front shocks that I have in my '79 Trans Am.
