I have a similar ANSA long 4-2-1 header on my 128 & like it. One recommendation would be to add/use an exhaust bracket from the underside of the transmission to the 2 side-by-side downpipes. This will help reduce the stress on the upper flange/pipes joint (caused by engine vibration, hard bumps, etc.) & deter any cracks at that particular joint.
I got them off ebay.. Just do a search for Fiat 128 Headers. They should be the 1st one to come up...
Seller's name is Andy Smith.
His company is Giant Killer Racing.
He mentions long history of 128 racing in several venues.
All that info coincides with a former Cosentino associate.
I sent him invitation to join in on this forum.