Aside from finding original replacements, I'd start with Borgeson. The link is to the early models, they list 57 and on also. They are not cheap, but I've never had anything from them that wasn't first class. You might also try a search at the ford barn or the HAMB, on the the social group sites, for the right year car.
You'll need to talk to them, and ask specific questions. Issues with the shift linkage, clutch linkage, headers, etc. happen. The advantage to a power box is you avoid the old technology and leaks of the ram. You'll also keep the original geometry, if the kits done right. I've got a Cavalier R&P in my bird. Works great, but it was a ton of time and money to work out the bump steer issues, get the "twitchy" out of the power rack, etc.http://www.borgeson.com/xcart/home.php?cat=117
55 bird, 32 cabrio F code