I have owned both and ran both in my 1967 Dart. I can vouch for the 15.9 with a dead stock 318 2bbl. I have seen it go as quick as 15.7. 273 4 bbl ran 16.2. Both with stock stall speed 904 torqueflight. Now the 273 I did have was a low compression 2bbl short block with a ldf4b manifold...