since it seems that most 3rd gen v6 cars come with 3.42 gears, according to haynes manual, could you use those gears in a 305? it's a lot cheaper to pull those gears out of a junk yard than to buy them new.

Sure, it makes good sense to me, just verify the ratio at the junkyard.

I'm moving this post to the Transmission section, it's not really a suspension section question.
David

You can use a rear from 82 - 02. Make sure that you pull the cover and check the wear pattern. Hold the ring gear and turn the pinion back and forth to see how much slop there is. Push the axles in and out to check for end play. At that time you can see whether it is a posi rear or a peg leg. Count the ring gear and pinion gear teeth. Divide the ring gear by the pinion gear. Example: 41 divided by 15 = 2.73, 42 divided by 13 = 3.23 and 41 divided by 12 = 3.42. Those are the three most common factory gear setups in the 3rd and 4th gen 7.5 10 bolt rears. You may get lucky and find one with 41 divided by 11 = 3.73 or even 41 divided by 10 = 4.10. The last two ratios would be aftermarket gears.