A pair of smoothbores is probably the best value tuning mod for a Nordwest. Allens Carburettors used to supply a Sudco kit suitable for XT600 Yamahas at the right centres so that they will drop straight on to the Nordwest intake manifold. Most people find CR33s are as big as you want to go.
To assist getting the carbs off and on for jet and needle tweaking replace the socket head screws that fasten the manifold to the head with hex head set screws. That way you can get an open ended spanner on, then remove the carbs and manifolds complete.
As I recall you need to grind a small amount off the frame to get the CRs in - an angle grinder is your friend. Also you need to remount the battery and starter solenoid to allow room for the CR intake stacks. Throttle cable routing is also tricky. Take care or you can suffer frayed inners at the carb end. I covered this briefly in my Nordwest Top End Rebuild article in GeN#8 on page 4 (page 2 0f the PDF): www.gilera.org.uk/gilera/gens/gen8.PDF
As to jetting, I misremember how mine ended up - over to Kano Nordi!
"Do not adjust your mind, there is a fault in the reality" 1957 175 Sport
I found I can fit the 33CR from the r-h side looking to the front with the carbs mounted on the inlet stubs, the opposite to the procedure for TK carbs, I did not have to grind anything from the top frame tube to give extra clearance or move battery box and starter solenoid, I'm using the standard air box with a modded intake rubber that had a cut where it clamped onto the TK inlet. I cut the intake rubber back from the carb intakes and bonded on a stretchy rubber membrane with 2 holes a snug fit around the CR bellmouths, easier to fit with the rear subframe dropped away the throttle cables are routed along the r-h side of the frame from the headstock and have a slightly shorter run than the standard TK setup giving a little slack in the cables, fit the cables to the carbs before inserting them in the hole(same for TK)72degrees used 150 main jets with a 604cc big bore kit but my Nordie is running fine and clean on this jetting with my Remus "silencer" it was loud before - it is now very much louder with very quick throttle response and feel a fair bit more lively my Saturno has a very similar 33CR setup with slightly cleaned up ports in the head but is pulling much higher gearing, which calms things down a bit compared with the Nordie