Pacemakers from Chris Mills in WA.
http://www.ebay.com.au/itm/HEADERS-H...-/321094150597
Tuned Length 4 into 1, Mandrel Bend = no kinks at curves.

Rod from MPH in Wollongong makes these which are better than the el-cheapo generics.
http://www.ebay.com.au/itm/Holden-HQ...-/360619561927
Tri-Y's with bolt up flange.

Always ask about head plate thickness, pipe wall thickness and tube sizes from primary to end.
Are they Mandrel Bend or Crush Bend? Heaps to compare on ebay.

If it were me, I'd fling the cast iron manifolds and really be heading for Pacemaker or Genies.
That said, I purchased a set of MPH extractors from Rod for my 308 powered HG (Tri-Y's) and they are good value.

No point having something cammed up with bigger valves and port work to have restricted with standard exhaust manifolds.
If nothing else the exhaust note is an improvement over standard.

Your car, your call. You asked for opinions from other users.