4.10 gears = 200,000 miles ?

Here's my 2 cents...

4.10 gears - at 80 mph with a T5 (overdrive 0.68), RPM would be 2950 with stock height tires. Not crazy RPM, but could get annoying over a few hour drive. Personally I would do a long highway run at no higher than 70-75 mph which would be 2500-2700 RPM. Drive 200,000 miles with 4.10s, most folks that put 4.10s in are not using the car as a long distance commuter. They may drive it often, but likely not 15,000 miles per year, probably 1-2000 per year max. So that would take 100-200 years to achieve that many miles with 4.10s installed.

6 or 8 speed conversion. Likely would not help much (if any). Look at the ratio of the highest gear. Rarely much lower than at T5 (I know a TR3650 has a 0.62 5th gear). For example, a 6R80 (very common for use with Coyote swap) has a 0.87 5th gear and a 0.69 6th gear, so slightly worse than a T5. The Ford 10spd has a 0.854 8th gear, 0.689 9th gear and a 0.636 10th gear (very slightly better than a T5).

Where the 6 or 10 speed conversion helps is due to the higher first gear you can run a lower rear gear (stock are typically 3.31s) which gives the lower RPM on the highway and still plenty of overall ratio at the start.

You didn't mention you were talking a V6. I assumed you had or were talking about a V8. If you are looking to make your V6 a little snappier off the line, sure go with 4.10s, they will work fine and help alot.