As long as the exhaust pipe has the same bends and lengths and the anchor points are in the right place it doesn't really matter if the silencer is round or oval (as Toyota original).
I have a custom made SS exhaust pipe on my SC and the box is round.
Maybe someone can confirm if it fits.
The price I paid for my SS exhaust was £280. So there is quite a price difference, but if it doesn't fit then you have the hassle or selling it on. Theres always going to be other opportunities, so if your're not desperate or its too far to see the exhaust I'll bid my time. You could always ask the vendor to email you some more pics and include a few of the dimensions such as from down pipe to first bracket, betweeen brackets and box and to the end of tail pipe.
I hope this helps you