Word of warning on your plans. Wingless sprints here in Aus are not 410 or even 360, but are a popular entry level class that run V6 engines. (From memory is the Buick V6 that was used in the local Holden Commodores here for a while.) So while the racing is still quite good, is not quite the same as the wingless racing you get back in the states.

Another option for you to look at is Western Springs Speedway in Auckland which has a number of big midget races over the Christmas/New Year period and attract a lot of American drivers down for it.

Winged 410 sprints are the biggest division down here, and the World Series Sprintcars is the traveling circus. Normally a lot of big name drivers, good car counts, and good racing. Also, Sydney Speedway and Brisbane Speedway (formerly called Archerfield) often have a few big name US drivers racing over the summer.

Most of the biggest races for Sprintcars are held in January to enable the USA drivers to attend.