[Jeff Lee]

The original 1969 SS head was based on the production head, ending with -558 and had rectangular exhaust ports. The standard -558 production head in 1969 shows 49.10 cc chambers while the SS only -558 head shows 57.00 cc chambers. So yes, the chambers were somehow modified on the 1969 SS only head.
1970 production cylinder head started with -291 castings which had the dog-leg exhaust ports and still retained the 49.10 cc chambers.

In Stock the 343 is rated at 275 HP and the 390 is rated @ 315 HP. They both look better in SS with 265 and 290 HP ratings.