I've never owned a '66. My experience has been with a '55, a '56, a '60, and two '63's.
If you already have the correct manifolds for a '66 (please check to make sure a 2 1/2" manifold fits your '66), and you wish to connect to aftermarket exhaust system, Dave has pointed you in the right direction.
Going back to the manifolds. In the full size Chevy the first small block to get the 2 1/2" ram horn manifolds was the 300 hp 327 (1962-64). Those manifolds (and the 2" ones for the 283 and 250 hp 327) dumped straight down and 'fit' the frame. One could swap the 2" and 2 1/2" manifolds. They also were 'ambidextrous' (one could swap right and left sides). I do not know if the different frame for the later years ('66 for one) impacts the manifold 'fit'. Certainly GM produced manifolds to fit the '66 full size small block engine. But were they 2", or 2 1/2", or both? Were they ambidextrous? I do not know.
IF you have Vortec heads in your 350, that introduces another 'fit' issue to resolve.
I noticed you live in France. I would not want to have parts shipped that were not right for my car. I'm simply trying to be helpful.