Compression Question

One of the other mechanics, putting on my exhaust said that my 340
in my 70, was of a lower compression being a Calif. car. I am going
to rebuild the rear- end and it was suggested to go to a slightly higher
gear due to the lower compression. I don't know if this is true, its not a
race car so I don't really care if the compression is lower. I'd just like
to know if it is a lower rated engine.