A great car as a proud owner myself I too find there are numerous issue when doing anything to this car as 1) they are so rare no stocks any parts and 2) because they are so rare no one knows anything about them.
I may be able to help you out here though. in all honesty these v8 engines will breath best will a seperate pipe from each manifold. There are numerous places that can make something for around 500. A 2.5 inch bore would be best and you would go from standard 201BHP to about 215BHP and will increase mid rev pull. Downside is it will be noisy.
An alternate solution for noise reduction would be a smaller 2 inch, joined after the manifolds into a single pipe. I wouldn't offer performance increases to mention since it wouldn't let it breath to it's full potential.
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