It sounds like a great age for him to be getting a tissue valve. At 41, I've had a much more difficult time choosing between mechanical and tissue. Your dad will most likely not need a replacement until he's 100 years old! I'm sure he will do fine.
Best wishes.
At age 77, I can't imagine anyone would choose other than a tissue valve.
Hope all goes smoothly. No reason to think he won't do fine assuming he is healthy in (most) other ways.
Surprisingly, this is remarkably successful surgery.
Best wishes to your dad. I am confident that he will do fine with the surgery and be up and around chasing the nurses in no time. I will keep him in my thoughts.