There has been a lot of conflicting info on this subject. Mostly because of the restrictive exhaust manifolds. I think the 67"s were more restrictive than the 68's. Thats why most 68's are rated higher HP......
They improved the ports on the 383 from 67 to 68...
I think it had something to do with the 340 coming out and they didn't want the 340 to beat the 383 so they had to improve the breathing of the heads...