This is the upper intake manifold gasket set. These gaskets must always be replaced when the intake manifold/plenum is removed.

This is the compact version of the engine thermostat intended for all Boxer engines (excluding single carb ones). 
The compact version thermostat is a replacement of the split-type on found on the earlier Boxer versions; Power assisted steering system required extra space for the steering pump therefore a new thermostat design was required. As a result the compact style version is fully backwards compatible with Alfa 33/Alfasud/Sprint/145/146 models twin carb or fuel injection.
Year 2019 Update: these thermostats are now almost non obtainable as Alfa Romeo has stopped production. After market brands like BEHR, Vernet, BGA have also stopped production so in case you are lucky to find a thermostat forgotten in auto spare parts shops, DO get it.

This is the thermostat on the engine.

Hydraulic jack makes lifting the front easier than the manual jack.

Place a piece of wood block on the crossmember center and lift the front just enough to get floor clearance.

Prepare the 17mm socket to undo the coolant drain plugs.

Coolant slowly seeps.

Allow coolant to completely drain to the container. In case coolant change is due then the right side bank coolant drain plug should also be removed so that the cooling system is completely drained.
 

Proceed removing the intake manifold/plenum. There are six (6) nuts on the intake manifold which must be removed. Access is relatively easy.

A socket extension is probably helpful to access the nuts on the inner side of the intake manifold.

A close-up of the nut on the inner side on the intake manifold.

Intake manifold right side fixing nuts.

Remove the intake manifold cooling hose gallery. 
The accelerator throttle has this clever design on order coolant warming up the air intake especially when the engine is warming up.

Undo the corrugated air intake sleeve hose/pipe connections clamps. These hoses are for the oil vapor recovery and extra air supply during startup.

Undo the two large clamps on either side of the corrugated sleeve (AFM and throttle side).

Using a small and flat tipped screwdriver, remove the throttle position switch connector.

Throttle position switch connector is out. Make sure that the retaining springs is restored on the connector.

The corrugated sleeve will be cleaned internally as oil residues accumulate over time.

Remove the ignition distributor vacuum advance hose just below the throttle.

Remove the two hoses. The upper hose is for the fuel pressure regulator vacuum pickup and the lower hose is for the charcoal canister located on the left side wheel arch.

Remove the hose on the front side of the intake manifold. The top hose is the idle speed adjustment nut and the lower hose is the auxiliary air valve downstream hose.

Intake manifold is out.

Do place paper hand towers on the intake ports to avoid catastrophic surprises.

Do the same for the left inside intake manifold ports.