Steve - why has there 'gotta be'?
what yer lookin for is actually
extra long bed parts, and there was very, very, very few made in the day, and waaay fewer still around and being restored - if you were a parts maker, would you invest in the time and equipment to make a part you may never even sell 1 of? and keep some in stock and get a number of distributers to stock, or even list them?
it's fairly recent that the regular long bed sides have become available - what you need to do is find 2 regular longbed sides [used, 1/2 or 3/4] and make what you need from the 2 - not hard, just a little painful, and waaay faster than lookin for a needle under the haystack!
Bill