I would just put the 20s on whichever end had more wheel gap. Id rather have 19s on the back though so you can have a taller sidewall and launch a little better. but im a dirtbike guy. we run 19s on the back and 21s in the front of our bikes. Same with race quads, littles on back and biger on fronts. And basically any other type of race vehicle that runs 2 different sized rims. The smaller rims on back.
Yeah I understand the taller tire in the rear I was just wondering what it would look like and you could also go wider in the rear with a 20. I saw somewhere that the 13-14 shelby comes with 19" up front and 20" in the rear.
Thats a super common set up with most sports cars, super cars, andy hyper cars nowadays. Corvette, Viper, Lamborghini, Ford GT, Koenigsegg ect...... all do that type of set up. My parent's 03 Corvette has 17" front and 18" in back.