No you're correct mate, but getting an accurate answer on tuned-length headers for an F5A/F8B will be a fairly exhaustive process. (Pardon the pun!) You're pretty much diving into the realm of trial and error unless you have the facilities to computer model exhaust pulses for a 3 cylinder vehicle, because I doubt that kind of research has been done before outside of maybe SuzukiSport in Japan.
If you want a generic answer, then just go for a "decent" length for each runner, I would say the rear of the sump is the furthest you want to go, but definately no shorter than the front of the sump.
As for my exhaust cracking, well the extractors were bolted to the block, but the crack developed AFTER the flange because the rocking motion of the motor, coupled with the movement of the exhaust system itself, was putting excess leverage on the flange.
At any rate I've fitted a flex-joint to the end of the dump pipe on my turbo now and have noticed a reduction in vibration/ringing from the exhaust in addition to reducing the risk of it cracking again.
