The 275/35/20 is 0.4" taller than the 315. I'd do either one of these options:
- 275/30 front and 305/30 rear.
- 275/35 front and 315/30 rear.
#1 gives you a little shorter front tire than the rear, whereas with #2 they are the same diameter. The problem with #2 is that there are very few tire choices in the 315/30/20 size.