Phoenix ones are a terrible fit,had one on my 2.5 Vitesse for a while and ended up selling it on e bay,i was lucky enough to find an original Mike Randal,
(Mike the pipe) 6-3-1 for sale so snapped it up,lovely hand made thing with sand bent tubes and proper joints.
My 1500 Herald has a 4 into 1 with a 3 inch collector and Vitesse system,manifold bought off e bay years ago,does it make a difference?..no idea.
Car is great to drive and quite torquey,it made 93bhp when rollered.
Steve