Do you have a copy of the "import approval" from when it came into AUS from the US - the date on that will help in terms of working our when the conversion was done
As someone else mentioned, there was a bunch of 65/66's converted and sold by Ford AU (around 209) but this is not yours. In the early days (60's) there was a requirement to convert cars to RHD for registration, can't remember when it changed to allow us to keep them LHD but I know of a number of 65/66/67s that went through that process (ones that came into AU back then)
Also, converting to RHD (just for convenience) was a thing even after the legal requirement dropped. Most cities had specialists that did it (some good, some not so good) but it has become less popular these days, I am guessing due to a combination of cost, people that can do it and maybe people are generally accepting of LHD (and for some cars, value is better retained as original)
I guess your best bet will be someone from the MOCA SA side seeing this post and recognising the car/history - maybe try contacting them directly? (
https://sa.mustang.org.au/), search of the forum might also help
cheers
greg