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They don't have to be San Marzano, sometimes its specified because they are such high quality and make an excellent sauce. If you cannot find them you can use canned plum tomatoes it will work just fine.
Probably not. I heard Lynn Kasper (The Splendid Table) once say that often canned tomatoes that claim to be from San Marzano often aren't. I wouldn't worry too much about your choice of tomatoes. I also remember a Consumer Reports blind taste test that chose Redpack as a the best.
I've made this several times and never with San Marzanos. It was perfectly delicious! It may be better with San Marzanos, but I would not worry about it for a minute if you don't have or can't get them. ;o)