After dropping the guys at Opel GT source www.opelgtsource.com a line to see if i could maybe adapt a header from the GT to fit the manta it turns out that one of the guys there had a manta years ago and kept the bits when he broke it up and has a 4 – 1 header from the manta and its nearly the same as the GT one. It 2 1/2 exit which is just right and looks like it hugs the engine, so it should solve the problem with the steering column hitting it like the old one did.
Now i just have to get Gaz to pick it up and bring it back next time he is over there!
Manta 4 - 1 header