Expansion chambers for a V8

[Hillclimber1]

2 years ago I installed an Evinrude -3.6l V8 into a race car using a CVT transmission.
Whilst I redesigned the original exhaust system using 2 cylinders firing at 180 degrees apart into 1 chamber, it ran OK BUT not with full power.
I am now looking at cutting off the original exhaust manifolds which grouped 2 into 1 and then the 4 x 2 outlets into a single outlet exposing the 8 individual ports.
However, on the car I have no room to fit 8 chambers , so need to design for 2 cylinders into 1, again choosing those 180 degrees apart.
I know this has been done by Radizon in Sweden BUT he had space for 8 chambers.
?????? Has anyone any ideas on the design of the headers that come from 2 ports and merge into one before it enters the chamber.

I am advised to design the chambers for double engine speed. i.e. Design for 13,000 RPM to run at 6500rpm max power point - Any comments.