I have an 80lb English Bulldog that needs to be chewing something constantly (or bad things happen).
We've found that a Nylabone Flex chew (made for 'Moderate' chewers) keeps him pacified... I think it's the springy/chewy/rubbery texture that stops him from just destroying it in seconds, but he'll sit and chew on it for hours.
Maybe give it a try..