American Autowire is the top of the line and most expensive. I have used their harness. In your case I would recommend This.
It is more circuits than you will need but that's OK and it's a good price. PM me if you need help with installing it. It will come with instructions but all these kits are "universal" type and you may need help with some areas.
The heater, electrical wipers and turn signals were all available in 1957 from the factory.
Speedway also has a 12 Circuit which will suffice if you are budget minded. I just don't happen to like the fuse box on that one.
In my opinion, you absolutely want a prewired fuse box type harness.
The Tri Five TF's are the best years and the 57 is best of them. I read that yours has a GMC cab. You could be in for trouble at DMV because a GMC VIN plate will not match the Chevy VIN stamped on the frame. In my state, it would not matter because we don't need to take the truck in when getting registered or titled.
It is quite odd that some "PO" would put on GMC cab onto a Chevy frame 40 years or more ago??????????? If that is what happened. What is the VIN? Does the title match the VIN?
Looks like you joined today and already have 7 posts. Good deal.