There's a company that makes new frames for 55 to 57 Chevys but they are repros of the original. If it wasn't a good frame to begin with they wouldn't be reproducing it.
If you rebuild the front end and add disc brakes you'll have a good riding and handling car. The 55 is built better than most "modern style" cars and never had a problem with reliability.
Keep your frame. It's the only one that fits a 55 Chevy. It's easier to bolt on new parts than to replace the frame. It won't be worth much with the wrong frame under it.