Preheat oven to 425. Add the fries to a baking sheet and season with some salt and oil. Toss to coat and spread them out. Bake to package instructions until browned around the edges.
Bring a pot of heavily salted water to a boil and add in the pasta. Cook according to package to al dente, about 8 minutes. Reserve 1/2 cup of the pasta cooking water just before straining it.
Meanwhile, whisk together the evaporated milk and cornstarch in a bowl until combined.
Melt the butter in a frying pan and add in the milk mixture. Bring to a simmer and watch as it thickens. Remove from heat
Add the pasta, cheese, and pasta cooking water to the pan and stir. Continue to stir until all the cheese melts. Be patient- if you added it all when the pasta is still hot it should eventually melt. If it doesn't melt, put it over low heat for a minute, and continue to stir, taking it on and off the heat every minute or so. Just be careful about overheating because the sauce will break if it gets too hot.
Pile up the fries on a serving platter, then pour the mac and cheese over them. Top with the bacon and scallions and serve.