On a stock engine car with no plans for upgrades, AOD, go 4.10s, you won't regret it.
However if you are going to swap to a T5, I'd do 3.73s. All of my prior foxbodies I installed 3.73s, all were 5 speed cars. The T5 with 3.73s is a whole lot of fun.
If your going to do engine mods and make more power I'd do a 3.27 or 3.55 gear personally. With more power you don't need as much gear, unless it's a drag car, but that is a whole different story anyway.
I've been debating this myself for my 87 GT. At some point I may bulld a 347 stroker, but still NA and not radically cammed or anything, I will likely go 3.73s. They really do make the car feel completely different, especially over the 2.73s my car has in it now.
I don't do much highway driving at all, mostly all country backroads and with the 2.73s, 5th gear is pretty much totally useless. I've never had a 3.55 geared car to compare it to, but I don't want to regret not going with the 73s. We shall see