The Walbro GSS342 is exactly the same as the GSS340 & the GSS341 except for the orientation of the inlets.
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The easiest way to choose which pump you need is to check your current fuel pump. The GSS342 and GSS341 fuel pumps are very similar and are probably interchangeable. However the fuel basket and the way the hoses are connected could make it a difficult install if you choose a pump that isn’t the same as the one you have now.
The GSS340 inlet is much bigger than the other two, and at the least will require a new filter sock (which I recommend you change anyway with a new fuel pump).
The electrical connections are the same on all three pumps but if the cables on the car are short, it may be difficult to use a pump that is different to the one you have at the moment.
Basically check on forums for your car to see which Walbro fuel pump people are using or have a look at photos of standard replacement pumps on site like Autodoc to see what it looks like or take the pump out of your car and check it. This way you know what you are ordering and will be correct and a straight swap for your existing pump.
Walbro GSS342 – Spot Fakes
Driven 2 Automotive has a good article here.