Headers for my 71 Demon 340.

I dunno man. For your various threads its hard to know what your main goals and uses really are. Your other thread made made me suggest TTI or custom to protect the bottom end.
Dougs are the best priced of the generally decent fitting ones here in the states. There's no step, so there ought to be a hair more room.
Dougs, and now TTI make headers for the earlier bodies (-'66) and those are much tighter to fit. Still, the people I know have installed them without dropping the K-frame. Those are labor intensive installs.

I'm sure there are guys in OZ under making custom headers. Stainless of course. See if you can message Hysteric on this board, he may know who.

If sizing is a question, see this post Someone school me on header size.

If heat reduction is important, then see if Zircotech has a operation in Australia. If so, then you can have your headers coated locally and maybe it works out better costwise. The Doug's cost significantly less purchased uncoated.