[BR3W CITY]

2 year old thread man, but for the sake of productivity;

What you can get away with with depend on how much you plan on lowering the truck, and to some extent, what method you use. In the front, your biggest threat with too much tire is either hitting the fender bolts, or rubbing the tie rod (or possible sway bar if you have one).

Your current tire is 27.4" tall all the way around.
The new fronts are 26.5" tall
The new rears are 28.8" tall.

Running a 29" tire in back is doable with plenty of combo's, but at full compression on trucks lowered 4-6", you may be getting close to that rear tub. I've got a stepside so my clearances out back are a bit different tho.

As for the front, that tire will clear pretty much any combo, as its pretty small. My concerns would be that a small tire on a 15" rim on the front of a very low truck creates even more lowering. This means that your oil pan and other bits are lower, in this case about an inch lower. So if you drop the truck 4", you'll be closer to 5" with the small tires. You also want to consider the way smaller diameter tires fill the wheel well. Too small and it gets very "90's mini truck" looking.