Hi Joe,
 I'm certainly no expert but for my LA .46 I ordered a factory (unmodified)  O.S venturi and a S.T nva from Tom Dixon.

 Not certain what the venturi size is but it runs great, even without boring the venturi out to compensate for the thicker spraybar.

 In your situation I would run it with the stock venturi first and see how it goes, you can always bore more out later.
 
 I don't think (without wanting to step on anyones toes) that there is a be all and end all venturi size for any particular engine, if it is in the "ball park" it should work ok.

 Depends on what style of run you are chasing.

 The optimum size can vary in regards to fuel composition, prop type and load, tank style and placement, muffler type and restriction, desired run style, weather, altitude, amount of power needed, etc etc etc.

 So I would start with it stock as a benchmark and work from there.

 The other option is if you have seen an LA .40 that runs the way you want, copy that set up to the letter!
 
 Cheers, Rob.

When you are trying to set the venturi size, run the engine up to peak rpm on the prop of choice.  That sets a bench mark for how fast it can run.  Then when you try different size venturis(effectively changing the throttle opening) you can tach those at peak rpm and know exactly where you stand relative to the original.  A lot of times you will have to change both the venturi and the prop to get the motor in a happy spot.

The .283 venturi with an ST needle valve through it is a decent place to start.  With the OS needle valve it may be a tad large.  A lot of folks use .283 on the 46 with the OS nva.   Depends a lot on the prop, plane, and fuel too. They all have to be adjusted together for best results.