Series 2a Gearbox problems

Hi all, can anyone HELP!!!!!! I have a 1959 series 2a landrover SWB. The gearbox is very grindy ie: it sounds like a shaft somewhere in the housing is catching. Whats the best thing to do, change the box or change the shafts and bearings. Also can you fit a s3 gearbox of a 6 cylinder onto a s2a if the bell housing is changed. And 1 last question as i'm still learning about landrovers. behind the housing for the gearbox is a small pipe which is dripping oil, i've hear dit is a pressure relaese for the engine but not sure, can anyone help.
WHEN I'M STOPPED THERES NO GRINDING, WHEN I ENGAGE GEAR AND SET OFF THE GRINDING STARTS.

Sounds like the box is stuffed then, allthough you might want to check the diffs, jack up the back of the car and turn the prop with the box in neutral and see if the shaft turns without grinding, then do the same for the front, if they both seem fine then theres summat up in the box or transfer box.
Does this happen in both 2wd and 4wd?
The 6cyl box is probably a bad idea, from what I know the box is the same except the bellhousing, but the housing is also the mounts for the shaft bearings and other bits in the 'box, it'd be a major rebuild to swap the bellhousing, allthough you might be able to use bits from the 6 cyl box to fix yours.
This any help?