4.10. 56's would be way too steep with the 6 spd. Most charts out there do not take into consideration the ratios of the newer 6 spd transmissions. You have an 8.5" 10 bolt.
Driving in 6th on the interstate he will be fine, but city is a different story. The final drive ratios in the 6 spd are a good bit higher numerically. 3.73's in a 6spd in 1st gear is like having 4.88's in a 4 spd. Rockstar, I certainly wouldn't attempt a gear install unless you/your friend are well versed. Gears are very tricky business and can easily fail and whine if not setup perfectly.I have the same transmission and with 8.5 / 10 bolt.....you be surprised how effecient the Od is! Everyone told me I would be pushing 3000 RPMs with the 4.11 @ 75MPH.....at 85 I am just hitting 2500 rpms....the truck isnt even working hard at that range.
Depending on how much freeway driving he does I think 4.56 would be fine.
I used this calculator when figuring my new gears and it was pretty much dead on. I use .70 for the OD ratio...
With 4.56 @ 80MPH it puts him at about 2500 RPM.......his engine can take that all day long.