Re: GM&O GSC 53' 6" Flat Car Build, Bloomington Shops 1951
You are correct about the configuration of the kit.
Or you can search in eBay for 'Walthers GSC flatcar'. Some old kits show up, but W must have reissued this product as "assembled".
But if you want the cast GSC frame only, you can buy any kit and throw out all the other parts.