Here are your 3 budget options.
Plug weld your hub and redrill to 5 stud. Simple. Uses standard brake setup or upgrade as per escort options.
Use ford 5 stud trailer hubs with rear xf disc brakes and your choice of calipers on modifed mounts. You will need to check with a trailer company to find a match of bearings to suit your hub needs. Have a look here
http://stores.ebay.com.au/Spinny-Things-Trailer-Parts/TECHNICAL-GUIDE.html_ for bearings and hubs or contact them for details.
You other option is to make a bolt on spacer adapter from 4-5 stud. You can then run post AU falcon wheels of just run with the extra track width of the spacer.
All are not ok for street but will be fine for speedway.
Shaun