I found one with exactly that length in the basement. Don't know what it came off of but it's a quality fork, has Campagnolo dropouts. Also found a Tange from a Diamondback that is long enough, but I still have the matching frame. Just mentioning it so you can be on the lookout for Diamondback forks as well.
Before I offer the fork for sale I will need to check the project frames. I have a 1989 3.0 frame in the queue that needs a fork but I am not sure 6.75" will be long enough. If that frame needs a longer fork I will be happy to sell it.