As a point of reference I cross-checked with the NHRA list of accepted manifolds for 396 cubic inch, 325 and 350 horsepower engines. The 1967 and 1968 engines had an approved casting number of 3883948 and the 1969 version of the same engines had the 3931067. I suspect that there are some detail differences between the two because there are differences between the years on small blocks as. Mainly the difference was in the use of an oil filler tube, a feature deleted on the 1969 manifold.
Disclaimer: NHRA's list, while very informative, can not be used as a final authority on anything pertaining to part originality. Political and economic pressures over the years have diluted the accuracy of many specifics substantially.