Advice on Cylinder Head cfm for 340 stroker

No mention was made on intended use, so I say go for well prepped Edelbrock Victors, or second choice would be Indy 360-1.
The Promaxx heads are sometimes said to be good castings and just need to be filled with quality parts as well, not the Chinese valves they come with.
All that said, I see you are in Australia so I would say go with whatever has the best local support. If you have good local head shops that use one of your original considerations, that is your best bet.