I bought my '21 RE specifically to drag race.. without any idea that the front brakes had been made even bigger. What I found is that there currently isn't and 18" front wheel that will fit over the caliper and the 20x6 type wheel doesn't have a drag type tire for it. So.. no good options either way.
I learned that there are no other braking options that will bolt to the '21 RE knuckle. The two mounting holes in the knuckle are what's been moved outward to accommodate the larger rotor. So.. if you want to run an 18" front drag skinny there look to be two factory type options:
First is change the knuckle to a standard pre '21 knuckle, purchase pre '21 RE/Hellcat rotors and install using all your other current '21 parts(including calipers)
Second is to convert to a 1320/SS small brake setup which will allow you to run any front wheel you want. You still have to change the knuckle out. Also, you do lose a bit of braking from this option(obviously) but nothing that's too bad. --- I went with this option since my car will be used for Friday night cruising and drag racing.
FWIW - changing out the knuckles isn't as bad as it sounds.