I am the owner of a 55 Tbird that has been in my family for 50+ years. The passenger compartment for 55-57 is the same, with the exception of the dash change in 57. Yes, apparently the manual seat (not available in 55) offers a bit more room. Removing the swing bar of the soft top (if so equipped) will allow the seat to go back a bit farther. It's possible to relocate the seat farther back, but there won't be adequate space for the soft top. You could also consider replacing the original seat with buckets.
There have been a number of complaints regarding a problem with the smaller aftermarket 56-57 steering wheel splines not matching up with the splines on the steering shaft.
The 55-57 Tbirds weren't designed with large people in mind. Fortunately, I'm 5' 8" and weigh about 150. If you ever have to work under the dash, particularly with the seat, steering column, and soft top in place, you'll come to respect how small they are. Before removing said items, it was necessary for me to lie with my back on the floor. with my legs draped over the seat back.
Rust? I haven't found it to be a problem on my 55, but my car was originally sold in the Los Angeles District, and it has never left California. Since it's been in my family, it has spent most of its life inside the garage, and it's not driven in the rain. I would, however, check for rust, bondo, and signs of collision damage for any vehicle that is more than a half century old.