In answer to your first quesiton ... any trailer repair place should be able to put the additions on that you asked for. Shouldn't be too expensive.
In regard to the petrol station question ... W.A. has a 500k distance between road houses on the bitumen (Port Headland - Sandfly) or at least it used to. The only other places I can think is the Territory ... the Barkley Hwy (500k's) has one fuel stop (breaking the journey 50/50 roughly) and the Tablelands Hwy (takes you from the Barkley Hwy to a place called Boroloola ... Gulf Country) has about a 400k strip with no fuel station ... Cape Crawford is your only one.
As for QLD ... Cheiffy and Kramer would be best to answer ... I've only travelled the interior and not the coast roads.
Don't forget that if your 600k limit is without a trailer then it will be shortened somewhat ... eg. my car goes about 550k's on a tank ... but with the trailer it's about 450k's. Just a thought.
The nullarboor has plenty of stops ... nothing over 300k's from memory.
(100th post too