Ya, they will work on a 15x6" rim. 10.5's are narrow so they should work ok. I am running a 35x12.5 on my 15x8's. I know guys running 42" TSL's on 15x8's.
You can always take your spare tire rim assuming it is also a 15x6" and have the 10.5" tire mounted on it and see how it looks. If you don't like it then buy new rims. If you like it run it.
Harley