you should make the exhaust flange bigger than the head port size, something to do with wave lengths and exhaust pulses :wtf:
they need to hit a ridge to prevent the exhaust gases bouncing back into the engine, so allow at least 2mm all round
a rough example; a 50x25 head port would need a 54x29 exhaust pipe :thumbup:
ideally the primaries and secondaries should be the same length, at a guess 350mm should be ok for each, so 700mm long