The ones on my 68 are three tube. A friend just bought some Doug's headers four tube for his 68, they sit slightly lower.

I don't know all the history of Doug Thorley headers but I believe "Doug's Headers" and "Doug Thorley Headers" are not the same.
Dougs headers is part of Pertonix and they call them 'the Original Dougs Headers' Doug Thorley Headers teamed up with Jardine Performance Exhaust and was bought by Summit racing. I don't know about the headers with the "Thorley" only badge and I don't know if the Doug Thorley three tubes are even made anymore.

As far as fit my Doug Thorley three tube fit better than my friends Dougs four tube. I don't know which would perform better at different rpm levels I've never seen any back to back comparison testing. I've read [can't remember where] the four tubes perform better at high rpm. More high rpm horsepower and the three tube will give better bottom end and mid range torque, but like I said I can't remember where I read it and have not seen any comparison testing so that could all be crap.