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Hi Rob. I have 2 sets of the same Ansa header you are using, so I am quite intrigued by your progress.
My day job for the last 25 years involves design and manufacturing of exhaust systems, and processing warranty claims to see why they failed. I've learned a few things.
Firstly I can tell you that you definately need a flex in the system. The engine is mounted on big rubber mounts and moves quite a bit. Something has to allow for that. And for sure you need a sturdy brace at the end of the header. Connecting the end of the header to the engine block or tranny. Otherwise the weight and vibration of the header is taken - with a lever effect - at the front header flange. Trust me, without a brace, it WILL - after time - crack.
Once you have the header well connected to the block, then you must realise that the weight of the rest of the exhaust system is supported by the body (with rubber hanger or springs). But because the header moves with the engine and the muffler is connected to the body, there then MUST be a flexible joint between the two. Fiat used a flexible joint with springs between the two. On the early cars this flex joint was between the manifold and front pipe. On the later cars it was between the cat and the muffer. Alternately, on the '74 US cars, and the euro cars (which had a lighter exhaust system with no cat), the entire exhaust system was bolted rigidly to the engine/trans. And NOT connected to the body at all. You must one or the other.
So in your case, the proper appproach would be:
Where your header ends finally to a single pipe (just after the last collecter), weld on a flange. (This will also make installation and later removal and service easier) ]. Make a sturdy brace that connects this flange to the engine block or trans.
Between this flange and the muffler, use your flex pipe. You can then support the muffer to the body with the stock springs or rubber hangers.
Then your proposed system should work great. Efficient design, low restriction, short pipe lengths. Should make great power !!
BUT.....although it sounds great in theory....there is a chance you MAY - I repeat MAY - end up with excessive booming or droning at certain RPMs. About 90% of cars use a muffler AND a resonator (they eliminate different noise frequencies). The only way to add a resonator would be to make your header exit to the right , go over and add a 180 degree bend to add one ( a resonator) across the car before another 180 degree bend into the muffler. This would add a bunch of pipe length and another bend to the system. And would completely change your header design.
Or stay with your initial configuration and you may have to try some mufflers of different construction to get an acceptable sound. Probably your best approach. After all, Fiat did manage it (on the euro cars) without a resonator so it SHOULD be possible....
Good luck. Cheers, Doug