Hello, coming in a bit late but... :
- Support the Niva so it is stable and as high as possible.
- Take off the knobs and the surrounds from the gearbox levers in the cockpit.
- Separate the two propshafts coming from the transfer box (take out the 4 bolts from each hub, (size 13 spanner) separate at the transfer box rather than at the axles, it will be lighter and avoids pulling out the sliding parts of the propshafts).
- Disconnect the two wires from the difflock warning light (they aren't directional, it's just an interrupter so the wires can be reversed with no ill-effects).
- Disconnect the doughnut (4 size 13 bolts).
- Disconnect the speedometer drive and the computer take on 1.7s.
- Take out the 4 nuts that hold the transfer box M8.
- Careful! It's heavy (28 kg) and difficult to manoeuvre, and also be careful of the levers as it comes down.

Done with the transfer box, on to the gearbox.

- Disconnect the wire for the reversing light (no need to mark these either).
- Take out the 2 M10 bolts (size 13 spanner) that hold the clutch slave cylinder (careful not to operate the clutch afterwards, it will pull out).
- Take out the 4 bolts on the exhaust manifold.
- Take out the 2 bolts holding the anti-roll bar to the body, 4 bolts in total. (M8 bolts I think, to move the bar and thereby get the manifold out ),
- Disengage the manifold.
- Take out the 3 bolts that hold the starter in.
- Take out the other little bolts that hold the bellhousing to the engine (I have a memory like a seive, there are 3 or 5 I think, an odd number and it's a 10 spanner that is needed).

At this stage it leaves the 4 bolts that hold the bellhousing to the engine and the 4 bolts on the cross-bar, so you must be ready to receive the gearbox.
- Take out the 2 big bolts that hold the bellhousing to the engine, you have to remove them from the engine compartment with a 19 spanner.
- Take off the 4 M8 nuts from the cross-bar (me, I hold the gearbox with my knee, yes I know it's not good. I'm a stikler for security, but here the only risk is to finish up with the box on your face, but it won't kill you. It's heavy but doable. For those who don't agree, plan on a jack).
- Just the 2 last bolts that hold the bellhousing, same 19 spanner, but be careful it's heavy (35 kg).
- Manoeuvre the gearbox, being careful of the lever.

There we are, the game is played. It can go quite quickly if it's well prepared, the right height, etc.
I was forgetting, like for almost all mechanical interventions:
- Disconnect the battery
- Stabilise the car and its wheels well
- Wedge the engine before taking out the box (potential risk of it rocking) and all other safety precautions.
For me the Niva is a hobby so something risk-free, at least for the mechanical part.

I hope I haven't forgotten anything.
Good luck.