This is less a matter of the cut and more a matter of how you're cooking it, unless you're willing to drop like $20 a pound on steak. Some easy ways to make cheap steak more tender are by using acidic marinades, pre-salting, and physical agitation (beat the shit out of it). Personally, for anything cheesy, I'd avoid the acid. And the physical agitation, because there's no need. That's all fine, because truthfully, salting and resting is the best way to tenderize a steak.
So: buy the cheapest cut of meat you can, really. As long as it isn't full of connective tissue. Salt it super well, all around, and then put it in the fridge, uncovered, on a cooling rack over a baking sheet. This will allow moisture to completely dissolve all the salt when it is drawn out, and then subsequently reabsorb into the steak: carrying the salt along with it. For this next step, we're going to reverse sear the steak. Don't ask me about it: read Kenji's article on seriouseats.
Just google "kenji reverse sear". It'll be near the top. Once we've reverse seared it, we can sear it like usual. Then, cut it up into whatever size bits you want and add it to your mac bake or mix it in or whatever. This is a pretty foolproof method to get the cheapest, tenderest steaks. Anything that goes wrong is very fixable on the next go around. Basically all you can get wrong is overcooking or oversalting the steak... and in this recipe, it's MUCH harder to oversalt,
because the salt is distributed through the steak. If the steak goes in as part of the mac bake, there are a couple of options... sear it and cube it, see how it goes added from the end. You could fully cook it in the oven then skip the sear. Or you could not cook it at all after dry brining and just cube it, or still cook it a bit and pull it from the oven early. You'd need to experiment with that, I can't comfortably recommend any specific answer to which one will be the best.