question about R18s

Kind of taco-adjacent, since the vehicle involved is a 4th gen T4R (for some reason, I can access TW but T4R.org is blocked here), but it's a general enough question and there's as much or more trail experience on this board.

I'm about to start into building a 2008 T4R V8 into a trail/rock rig. I love my Tacoma PreRunner and have taken it to Moab, but the 20" shorter wheelbase on the 4Runner and the extra low-end torque of the V8 have some definite functional advantages.

The rig I'm starting with has 18 inch rims, and I'm wondering if 33-34 inch tires on 18s will have enough sidewall (comparable to 31s on 16 inch rims), or if I should look at stepping back down to 17s or maybe even 16s; since it's got 4 wheel disc brakes sized for towing 7k lbs, I'd need to see what will fit over the brakes, but I'd assume that I could probably go down to 17s at least. Is 1/2" of sidewall going to make a big difference one way or the other?