Most common rough-in is 12 inches. Toilets can be purchased in 10, 12, and 14. I am not a plumbing supplier but if I remember each size is designed to allow at least .5 to 1.5 inches of space behind the tank to the finished wall. There is no such thing as an 8" rough-in. Hopefully you have a pier and beam foundation so you easily reconfigure the rough in.
You need profession help.