What's the best resource to swap/trade 10 bolt for 12 bolt on my '68? What ratio would you suggest for 396 with turbo 350 automatic?

The best resource?Idon't know.Maybe your local speed shop or someone else who know's how to set these rear's up,they can get pretty involved.A swap meet may be a good place to find one cheap that needs to be rebuilt. then have it built to your spec's (hardened axle's c clip eliminators etc. the list goes on) However buying one at a swap meet that is supposedly rebuilt is a roll of the dice, you may get a good one and maybe not , then chance's are you may never see this guy again. Too risky for me.Another area to look at now.Do you have single leaf spring's or multi leaf?The welded on spring perches on the rear are different for each set up as are the shock plates and the shocks themselves. As far as the ratio is concerned,this all depend's on how the car is going to be driven.I basically have the same set up A mild 396 and a 4 speed your turbo 350 has the same third gear ratio as my 4 speed's fourth gear, they are both 1 to 1. I now have 3.73 gears this gets a little loud and annoying on long cruises ot the interstate at 70 mph plus. knowing what I know now I would have went with 3.55's,a very good all around gear I'm sure most will agree.Then again if your looking for stop light action,go with lower gear's.Mash the go pedal and start steering yeehaaa that's ton,s of fun too!The neighbor's will love ya. lol Hope this help's. Russ.

Have you considered an 8.5 10 bolt instead of the 12 bolt? They are much easier to find, a lot cheaper and almost as strong when built right. They can be found in '71 and later Novas and other X body GM cars. The 3.42 gear would probably be best in this rear end.

Then there is a new 12 bolt from Moser, or one of the other rear end manufacturers. They start at around $1900, but are even stronger than the original 12 bolt.

I'm preferring a 12 bolt. I do have multileaf springs.

You might be able to find a good original 12 bolt, and you might want to consider the Moser. You can get the Moser any way you want it, where you will probably have to settle for whatever you can find in an original 12 bolt.