Will a 308 shell fit in the chamber. If it will try and get your money back because it is dangerous and may have already cut into the bottom of the read sight mounting holes. Firing a 308 in a Contender would probably ruin the frame and make a grenade out of the barrel in one shot. I saw a 357 barrel some one had re-chambered to a 35 wheelen and called it pocket dynamite , probably a true statement.
I agree that 308 would be dangerous pressure wise, but it will not even be close to the rear sight holes. The 45/70 is larger in diameter and is chambered in Contenders.