275/40/17 on a 17x8? is it okay?

I'm lookin at a 275/40/17 for my 68 cuda. I currently have a 245/45/17 mounted on a 17x8" wheel. I know I can fit a 275. Problem is, a 275/40 is recommended for a 17x9 inch rim, not a 17x8. What if I mounted it on a 17x8? Anything bad? Can I do it? Is it not safe? What are your thoughts?

The tire place had a little bit of a hard time mounting my 275/40/17 on 17x8 rims.

I'm just running them this way for mock up. But they seem to drive fine.