Choosing exhaust 77br Derbi

Hello I’m building a 77br Derbi Senda with these parts: stage6 77br cylinder, Mvt dd ignition, stage6 rt viton reeds, pwk 28mm carb, jasil evolution crankshaft, voca pwk intake and I’m wondering if o should get a Mvt s race 70-80 exhaust or a Mvt s race 85-100 exhaust.
Thanks