I'm not sure what range of years have interchangeable axles, but the only way I know to check gear ratio is to turn the rear wheels and count the revolutions of the drive shaft - might be hard to do with truck in a junkyard.
I have a 4:10 C20 with a 454 and it sucks money out of my wallet. I assume that you are aware of the lack of fuel economy, and lower top end mph when on the highway. When I was growing up, we called 4:10 and 4:56 low gears - guys put them in their 1/4 mile cars. Some call them high gears because of the high numbers.
