I would just go to a muffler shop and have them cut, bend and bell a piece of straight two and a half inch pipe to fit the rest of your existing exhaust. Probably cheaper than any other aftermarket pipe that you will have to make your exhaust fit it.
Problem will be in finding two and a half inch exhaust manifolds as only the drivers side with the generator mounting bracket cast in place is reproduced. Most buy two and saw off the passenger side bracket (you can grind it flush) so that it looks stock when installed in the back of the engine on the passenger side.
Don't forget to install a pass through in place of the heat riser valve. You use the same gaskets (donut and triangular flat one with stainless steel bead) as the regular exhaust.
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.