The only "need" for the king lead is dictated by the position of your dizzy and ignition coil, so move it closer and it can be shorter.
Misfires can be caused by all manner of things on a cossie, even routing the ht leads too near to the tps/phase can cause it due to electrical interference.
Don't make the mistake of assuming the very overpriced overhyped coilpack conversion is a cure all, a better coil with more energy can sort a huge amount of issues with blow out.
Using plugs like IK27 etc can help no end, as they need less power to fire, but I'd have a look at everything else first.
tabetha