I am currently running 275/70r18 Duratracs on 18x9 TRD Pro BBS and plan to keep that as my winter set. Would a 275/60r20 AT fit? Would I also need to look into bump stop modifications as well just by virtue of running 0 offset?
Can’t help with the fitting of 0 offset wheels, but an easy conversion for rough estimates of tire sizes is that for every difference of 5 in the sidewall profile you can go up or down one inch in wheel diameter for similar overall width and diameter (decreasing sidewall allows for increased wheel diameter and increased sidewall requires decreasing wheel diameter)
275/70R18 will be close to 275/65R19 which would be close to 275/60R20; they are all a roughly 33” diameter tire with a 10.8” width