Are there any checks on a gearbox before putting into car?

[246gts]

Slightly off topic but I have just got my frogeye running to find that the "reconditioned box" jumps out of reverse, leaks oil and is difficult to engage third. It has the smooth case box similar to the Minor, and I have two other of these boxes at home but dont want the hassle of taking out engine and box too often.
How do you choose a good one???
Dave

[mike.perry]

Check the free play on the front shaft.
Select each gear in turn and turn the prop saft coupling and feel/listen for any rough spots
Remove the side cover and inspect what you can see for wear or damage.
I expect that someone who knows what they are talking about will give you some more info.