An 8.5" width rim with +12 mm offset has a backspacing of 5.25", so the inside edge of the wheel is 1/4" further out than stock and the outside edge is 3/4" further out, (1/2" due to the width and 1/4" because of the backspacing).
With the 34.45"x12.13" Trail, you will likely have to remove your front mudguards, and heat and reshape the fender liner at the pinch weld area, but it's a toss up on if you will need to do the actual pinch weld mod, dependent on your year and model, and the Titan to Titan variance. I think that you will most likely need it though, especially since they are an M/T with the bigger lugs at the transition from the tread to the sidewall.
If you are willing to do the pinch weld mod, (see the link in my sig), they'll fit for sure.
Also, for less likelihood of needing aftermarket gears, and probably no pinch weld mod, (but probably still have to loose the mudguards), the shorter LT305/55R20 33.4"x12.2" (on an 8.5" rim), would be the better choice. Sidewalls get pulled in on the narrower than spec sheet's measuring rim, but they probably have a little wider tread.
