Usually, processed cheeses have enough extra sodium citrate (or other emulsifier) to help make a smooth melt from anywhere to half the processed cheese weight up to equal weight.

Mind you, there's still the tradition factor. Making the bechemel gives a different taste and texture that citrate. So, even if you decided to just keep the sodium citrate around by itself (it's cheap and easy to get), it won't be the same end result in terms of what people expect of a good, homemade mac n cheese.

I'm okay with bread crumb topping, but I prefer a good shredded cheese across the top so that it gets a little crust to it by the end of the bake. The topping is usually going to be cheddar, or a cheddar/American mix (not Velveeta, and only an actual cheese American as opposed to fake), with cheddar, Colby, and either swiss or muenster as the preferred inside cheeses. Maybe add some jack or pepper jack for the right crowd. Can even crumble up some bleu in the mix instead/as well.

But the exact cheeses don't matter as much as that base bechemel. As long as it's smooth, the cheese sauce is going to be the right texture.