I do clods quite often and have not found them to be tough at all. I usually cut it in half , the narrow end makes excellent pulled beef and is very tender. The larger end I slice like brisket and my friends and family like it every much as bit as brisket. I do inject sometimes with just broth and other times with Butcher Block prime brisket injection. I take them to 205F or beyond until probe tender and rest in a cooler after for about an hour. I am actually going to be doing another one on Sunday.
I can't think of much of anything that is tough once it's taken to 205°.
Somebody earlier suggested the OP take a shoulder to 135° for "slicing like a roast. That would be tough, unless sliced very Thin!
Bear