Mark,
The 68 L78 carb is a Holley list number 4053 and the Chevrolet P/N is 3923289 DZ. They come up on ebay all the time so you should be able to find one with a date code you need. Just do a search...
This carb is also being reproduced, so if you don't care about a correct date code, you can go that route.
Ed