If you must change gears, 4.10 without a doubt. Don't be scared by the fuel mileage comments, as most of it is hearsay. If you're going to do work, you're going to do work and you will not get some magically high mileage figures. A 4.10 isn't going to require you to go into the oil tanker business as a side line. What you will find is the mileage might be slightly lower lower overall, but with more consistent figures from empty to loaded. With the option of having an overdrive available, a 4.10 is nothing to fear. I've run this size truck as my daily driver since 1970 with 3.73, 4.10 and 4.56 axles. The 4.56 was by far the best and would give me a consistent 15mpg, empty or loaded. Granted, there are other factors involved with mpg figures. Since you have the lighter 4L60 transmission, the 4.10 would also be easier on it. I might also add that the numbers might look rather big, but the difference between a 3.42 and a 3.73 isn't much and not really worth the cost of upgrading.
The flip side of things now.
1) Your speedometer and shift points will be off. GM probably will not recalibrate it unless such an option was available as a stock item that year, and they still might not do it. Then you will have to go aftermarket.
2) It's possible that your rear carrier may not physically support either gear.
3) Depending on your supplier, the final installation might end up being noisy even if it's set up well. OEM gears are lapped and mated much better than aftermarket.
4) A slim chance you will need additional cooling.
