Basically you'll have to get tires that have the same height/diameter as the tires you currently have. There will be more sidewall to the tire, but the wheel gap will be the same and the speedometer will still be the same. Using a tire calculator I found for the rear you can go with a 305/35/18 and it's the same width and almost the same diameter/height, just 0.4in taller than you have now. Or a 295/35/18 which is 0.1in taller, if you don't mind giving up a little width. For the front, what matches the 265/35/20 you currently have is a 265/45/18 and is only 0.1in taller. What's weird though is on your current set up, the front tires are over an inch taller than the rears, which could become more apparent by the increase in sidewall. Personally I'd recommend a 265/40/18 so it would be closer to the height of the rear tire.
If you want to play around comparing different sizes here's the link:
https://tiresize.com/calculator/ just click on the 'tire comparison tab.'
Hope this helps!