Horse power drop using early A 273 manifolds

In '68 and '69, the 273 and 318 used the same exact manifolds, so, how can there be any difference? If your question is whether using the '64-6 LH manifold instead of a '67-up LH manifold, I doubt there would be much difference since the 273HP engine had 235 HP vs. 230 for the stock 318. Now, with engine mods, and/or high RPM, you'd need some dyno testing to see what differences there might be.