Finch, here's some history of 1969 chevelle engines, fr
There were four 396 engine options offered in 1969. (#1) The base 396 engine was rated at 325 HP for the basic SS package (in '69). This was also teamed up with the 3 speed transmission as standard issue, the four speed and automatic were an option. (#2) The 350 HP version was an option, as well as the (#3) 375 HP (Known as the L78).
The original '69 brochure for the Chevelle did not list the L78, but the El Camino brochure did. If you had a knowledgeable sales person, he would have been able to order the L78 engine for you in a 1969 Chevelle SS. The other (4th) option was the L78 with Aluminum Heads. This was the rarest option. The HP was not rated higher, but the weight was a considerable improvement over the cast iron heads. Both versions of the heads on the L78 were big rect port heads and both the 350 HP and two 375 HP versions had an aluminum intake, which the 325 HP version did not come with. the 325 HP version and 350 HP versions had the smaller oval port heads as well. Later in the 1969 production year the bore of the 396 changed to a 402, however, the car was still called an SS 396, then the 454 arrived in 1970. All SS 396 Chevelles were 402 CI in 1970 with the SS 454 as an option.