Welcome to the addiction !! To support what LAB said, it is definitely a 1975 mk3 850. I have a '75 as well and my s/n starts with 330. The '75 MK3 interstate and roadsters differed only in the tank, seat, and side covers. So, along with that roadster tank, you should also look for a roadster seat (won't fit with the big tank), and a set of matching steel side covers. Looking at your pictures I noticed that your grandfather put on a set of pea shooter silencers. Certainly acceptable, but your bike came with what is sometimes called "black cap" silencers. It would be pretty cool if you could find the original set. You also appear to be missing the chrome grab rail that fits on behind the seat.
A word of caution: even if she was good to go and you could start her right up; don't ride the bike, even a little bit, with those tires. They look to be original and could potentially throw you and the bike on to the street, and that usually hurts (a lot).
Good luck and stay in touch.
Cameron