I thought I might be having to go down the road of replacement too, but the radiator shop quoted a little over $200 for a re-core job so I'm going that way for now.
But, if I need to, I would go to these guys
(Brassworks) they have a specific listing for the '55-'59 Chevy and GM trucks. $300 is a bit pricey, but they seem to be very well made and if you live somewhere that you get frost - well, it is probably worth it.
