Hmmm...
This means you didn't measure the available mount spaces in your case before buying a somewhat-large heater core. DD even gives you the outside dimensions (11" x 6 1/8" x 2" ) so you don't have to guess about how big the tanks are.
If nothing else, it's a good lesson in "measure twice, cut once".
My guess is that you'd have to put this on the bottom - or on the top if you're willing to give up an optical drive bay or two. If this wasn't a viewport case, I'd think about putting it on the inside of the removable (left) panel - and you
could put it on the outside of the right side panel, vertically near the front, pulling air into the case.
For those wondering, but to lazy/busy to go find product links, here they are for the
case and the
heater core