Does anyone know where I can get rebuild kit for steering box, or if not available, least expensive box to replace the stock.
The best box you can get that's a bolt in is one from a 59 Ford. Installed one in mine and it's one of the better changes I've made. A couple items to note is you'll need a 59 Pitman arm for it and if you use the 57 steering column jacket you have to install a separate button for the horn (steering shaft is solid on a 59, nowhere to run the wire). If you can get the steering column jacket along with the steering box your miles ahead.
The 57 box is the old design, same as used in earlier cars and trucks and isn't the best. If you do a search through the forum you will see the many post regarding this, The new design came in 58, I use a 58 box. It also requires having the 58 pitman arm. Only major problem is the horn wire as mentioned.