What to do if there are no NAVAIDs or GPS?

I’ve asked this earlier, but after a bit more playing I have some more clarity around what it is I’m asking.

I want to fly the DC-6 from Melbourne to Perth without GPS. I can confirm that there are no routes where you can get positive location fixes via NDB or VOR the entire way. There are airways that are bounded by NDB/VOR but their range does not cover the entire segment.

What is the right approach here if you want to fly IFR (weather etc)? Are you meant to fly IFR on the segments that you can (ie, Perth and Melbourne) and switch to VFR in the region’s you can’t (notifying ATC)? Or is there something in IFR rules that allows you to continue to fly without a constant fix? Or are you meant to request to be vectored?