Good advice, Jim. Just a few quick notes...
The Tire Rack is showing the same March 8 arrival date for the rear 265/35-18 size.
I know that was true of its RT-615 predecessor, but is that still true of the RT-615K?
I have only run the 615 not the K. But my understanding is it is the same tire but slightly different compound (old ones would get real hot and greasy on the track - more of an autocross tire than road course). The tread pattern looks identical.