Header lengths? for UK enthusiast!
check out the Exhaust Science Demystified section by David Vizard
Just go twin 2 1/2" from there back and you will only be down 5-10HP max in the top end.
However my engine I needed to make my own pipes as no standard ones would fit, I went 1 5/8" primaries, 2" secondaries and 2 1/2" collectors.





