Look at the tire specs on the website of the tire manufacturer.
For example, my favorite tire that will fit your truck on stock rims and look great while being a little bigger than stock-size tires is the Michelin LTX AT2 in size LT285/65T18. If you go to the Michelinman website and find that tire and click on specs, you'll see the size LT285/65R18 will fit on rims between 8" and 10" wide. Your rims are 8" wide, so there's your tire.
That tire is 32.56" diameter, or what most tire guys call a 33. It has 639 tire revs/mile, so it's a little taller than the stock tires that have 656 revs/mile. And at 285mm, it's a bit wider than your stock 265mm tires too. If your speedo is accurate now, it will be 3 percent slow with those tires if you don't have the speedo calibrated to 636 revs/mile. IOW, 74 indicated will be 76 on Officer Bob's radar gun. So if you thought you were safe at the 74 MPH ticket limit, your tall tires caused you to get a ticket for more than 5 MPH over the limit. ;(
Michelin LTX A/T2
Michelin LTX AT2 does not come in huge sizes that will be too big for your rims. But others do. For example, if you go to Toyo's website and look at the Open Country M/T specs
you'll see bigger ones, up to the 38X15.50R18LT. That puppy requires rims a minimum of 11.0" wide, and they can be mounted on rims that are 12.0 and 13.0 inches wide too. But not on your 8" wide rims.
And you'll also see the LT285/75R18 which requires rims 7.5" to 9.5", including your 8" wide rims. But because of the 75 instead of 65 profile, those are 35.1" diameter with only 591 tire revs/mile, or taller than most so-called 35s.
Is most informative about what is the max size tire you can safely mount on your rims. Look up the specs for the AllTerrain T/A KO. You'll find that the minimum rim width is:
265/65R18 = 7.5"
285/65R18 = 8"
305/65R18 = 8.5"
325/65R18 = 9"
So the stock 265s are fine. The oversize 285s are fine. But the 305s are too big for your 8" rims.
But you'll always have rebels that say "I don't care what Mama don't allow, I'll put the big donuts on my rims anyhow!" So they break the tire makers rules. I'm as guilty as anyone. I found that if you don't go more than one-half inch less than the approved rim width, then the tire will probably mount and ride and wear reasonably well. The only problem might be rubbing on a stock suspension. So if you are a rebel and would really like the biggest tires that would work on your stock rims, then you might want to try the 305s. They might work, even though BFGoodrich says you shouldn't do it.
And by the way, if you break the tire maker's rules, then don't expect the tire maker to honor their warranty.
So learn to find the tire manufacturer's website and then find the specs for the tires you care about. The proper wheel widths will be included in the tire specs.