Soundbiscuit,
If you're interested in buying a new Zenith carb, you can purchase this brand new straight from Zenith Fuel Systems. It's not cheap, but it will do if you're really looking for a new one. And it comes factory adjusted as well.
This is their new website. It used to be called Autotestsys.com, but all of their items are now on here. The Zenith carb for your 1955 GMC 248 is part number 14275. I have a 1955 2nd Series 248 and this is the exact carb. Their new site does not have all the pictures for their carbs yet. Their old site had the pictures for all of their Zenith carbs.
http://norfar.com/collections/carburetors/products/zenith-14275-carburetorA.R.
