Watch this video Shay at the 1:44 mark you see the exhaust system laying on the ground. My header isn't as elaborate as the 1:1 but it does have twin pipes leading into the silencer.
I like the Revellogram mentioned above. Its 1/24 scale but I have used it in several 1/25 scale builds.
Here it is in an AMT '66 Falcon which is 1/25 scale.
Wasn't happy with the header, too much if hung under the car exposed to damage that rally cars are prone too. Fabricated this:
Reminder this was the before header:
I'm building the Jean-Claude (JC) Andruet car, chassis #G11
https://www.ewrc-results.com/carinfo/98-fiat-131-abarth/?car=3928
Relying on this book as reference as well as the Classic Rally Models - photo CD and internet research.