Dave Tyler based in Royal Wootten Bassset has templates and formers for these. He made me an external manifold for a four cylinder car. 07813 204343. You would have to either have a car he can work on in his workshop or be very precise and detailed with measurements. The angles vary and whatever is made has to fit not just the engine but through the slot in the bonnet panels.
Hi Berndt, as Paul has said Dave Tyler is best bet.... when I stopped making them he had all my layout boards and templates.He can make off the car but would need dimensions as appropriate to your car/bodywork/engine. Also best man to bend up 2" dia. tail pipes" Has a website Dave Tyler Exhausts. workshop is near Royal Wotton Bassett. FOZ