Rims must be the same width as stock, so no rears on the front in stock class.
Mark, correct on using the 5mm spacers. I need to look at my math, but I believe that Breytons GTS-Rs have a slightly smaller offset than stock, which allows you to use a 10mm spacer to get you EXACTLY at the .25" offset variation allowed by SCCA solo-stock rules.
I've been using R6s with pretty great results (as you've seen). R6s last longer and have a wider operational temperature range than A6s. I'm estimating I'll get 100 runs on my set of R6s which had about 70% tread on them when I got them. 245's also seem easy to find on eBay which keeps the price down.
A6s, of course, have the best ultimate grip, but you might only get 30 runs out of a set. Possibly less, this car destroys front tires.
My advice, buy R6s to beat up on local competition, buy A6's when you ship your car to the mainland for Nationals
I'm running 245s square on my car. They do rub in the front (shock tower and on the front control arm at full lock). It's not enough contact to cause damage to the car or the tire.