A 833 od

A stock 340 will be a piece of cake. I know of 833OD's that are run behind moderate 440's without issue.

They are only weaker because of the aluminum case and clearance on the counter shaft. The gears themselves are just as strong. If you're really concerned about it, you can have the aluminum case bushed for the countershaft. But more than likely it will never be an issue.

There are numerous threads on this topic, and the conclusion is usually the same. Unless you're racing frequently with big horsepower, you probably won't hurt one.