Here is an excellent earlier thread on this same topic
1952-5700 2ton Brake Return Spring Equivalent that offers some previous advice on possible alternative springs.
In reading through the conversation, it looks like the Raybestos H500 spring kit is the closest approximate for the original GM springs. Fellow Bolter "3B"
made a suggestion on how to possibly make these work for your application.
There are a couple of these brake spring kits
here on eBay at the moment that you may want to try and pursue based on the information in the older thread.
If not, post a want ad in the Swap Meets
Truck Parts Wanted forum.
Good luck!
PS - Springs from donor Chevy 4000 thru 6000 Series (and similar sized GMC) trucks from 1951 to 1957 will fit your truck.
