Tire width has zero effect on gear ratios. Two totally different systems. What you have to try and keep constant as much as possible is the total tire diameter.
Play around with this:
http://tire-size-conversion.com/tire-size-calculator/
Now, between 285 and 305 there is about 1/2" in height difference so you'll notice your speedometer will be off slight but this won't be a big deal.
I hope you know that putting 4.10s in your car will affect your speedometer significantly if you don't get it tuned, at a true 60 mph you'll be seen 75 ish on your speedo. (unless you've already done this)
The thing you do have to worry about is that your wheel needs to be wide enough for a 305 tire or else you're messing up the intended geometry of the tire by either bulging or stretching the tire. a 19x9 wheel accepts 255 to 275 with a good fit (285 fits but not encouraged), a 19x10 wheel asks for 275-295, etc. Keep in mind tire models have varying widths.
Wider tires doesn't always equal more traction if you use it on the wrong wheel size. With 4.10s I'd probably do a 295/35R19 tire on a 19x10 wheel out back and 255/40R19 on a 19x8.5 or 19x9 wheel up front. Americanmuscle puts 275s on their 10 inch wheels which is on the narrow end.
But as Dirty D said, you can counteract the added speedo change with putting the "wrong" height tire in, but it may look funny and won't really get back to normal until you put a quite tall tire.