What's on your truck now? Just go with that size if it looks ok to you. I'd probably go with the 285/70's myself.
Here's something that should help you:
Plus Sizing Calculator
Plus-sizing has become a key tool for tire dealers succeeding in the high performance tire market. Still, many dealers remain concerned about tackling plus-one, plus-two, plus-three or greater tire fitment changes.
The basic rule of thumb with plus-sizing is: the overall diameter of the new tire should not be significantly different than the overall diameter of the old tire. A significant change in overall diameter - .75-inch or more - means the vehicle’s speedometer will not be accurate, and more complicated adjustments will be required.
Many tire makers, such as Yokohama, Pirelli, Bridgestone/Firestone and others, have developed easy-to-use plus-sizing conversion charts - like the one below - that make it easy for dealers to select the proper tire and minimize impact on tire diameter and the speedometer. Here’s how to use this chart:
Using the horizontal top row of this chart, find the current tire’s aspect ratio
Using that column, go down to the current tire’s section width. This is your starting point for all Plus Sizing calculations using this chart.
Section widths along the same horizontal row, including their corresponding aspect ratios, are the Plus Zero size options and will fit the customer’s existing wheel. (This assumes that the customer’s current wheel is, in fact, the correct size. If a Plus Zero tire change is desired, consult tire/wheel fitment guides to make sure the current wheel is the correct size.)
For Plus One Size Option - From your starting point, move up one row and then to the right two columns. This will give you the correct section width and aspect ratio for a Plus One fitment. Example: If your starting point is a 60-series tire with a section width of 225, the correct Plus One Option is a 50-series tire with a section width of 235.
For Plus Two Size Option - From your starting point, move up two rows and then to the right four columns. This will give you the correct section width and aspect ratio for a Plus Two fitment. Example: Current tire is a 60-series with a 225 section width, then the Plus Two Option is a 40-series tire with a 245 section width.
For a Plus Three Size Option - Again using your starting point, move up three rows, then move to the right six columns. This will give you the correct section width and aspect ratio for a Plus Three fitment. Example: Current tire is a 75-series with a 205 section width, then the Plus Three Option is a 45-series tire with a 235 section width.
Once the Plus Sizing Option is selected, check the load carrying capacity of the selected tire size to make sure it is equal to or greater than the current tire.
If the Plus Size Option does not have adequate load carrying capacity, it may be necessary to either reduce the amount of plus sizing (if the lesser tire provides adequate load capacity) or increase the Plus Size Tire Option by 10mm to achieve the proper load carrying capacity.
Consult your manufacturer’s tire fitment guide or databook for load carrying capacities and other related details.
Warning: Any Plus Sizing tire fitment change requires a new wheel in the correct size for that particular new tire size