35's to 37's. Easy. 39-40's with some trimming.
I ran 35's on my 87 4runner with no lift, and trimming the fenders and firewall a bit. But honestly, I didn't have to trim much.
You will certainly like driving the truck a lot better with gears. But, which gears you get will depend on what size tire you want to run.
Driveshafts may or may not need to be modified. It will depend on what your brand of lift recommends. If it doesn't recommend modifying the shafts, or says nothing, give it a try without modification, and then have them lengthened or shortened as needed if a problem arises.
I ran 4.88's with 35's and a 22re, and it was pretty good. 5.29's would have been nicer on the hills, but the 4.88's cruised pretty well on the flats and downs, so it was a trade off.
The larger tire you put on, the more stress you put on the steering and axle shafts. If you have any issues, I have a LOT of experience with the IFS on those trucks, and I can point you to places you can upgrade, or things you can do to strengthen parts, if you need.
Baby Beast 2- 1999 4Runner SR5
Baby Beast -1987 4Runner SR5-Gone, but not forgotten