I had a 270 with 270,000 miles on it before it went in for a rebuild. They are good strong engines.
The lifters should go back in the holes they came from. You could have the lifters resurfaced. The lifters do rotate So I would imagine it wouldn't be the end of the world if you got em mixed up. If you are woried about it, you could have your cam reground, and get a new set of lifters relatively inexpensively.
