The tall adapter should fit with the stock air cleaner if on an L16. L20 should be too large though... Tuesday I will know if the Offy manifold will allow a Weber w/stock box on an L20.
It depends on how their store is setup. There are two options... one is to set the store to show "sold out" when inventory is reached and the then you can set it to oversell.
Yes you need an adapter to run them. You can get them on eBay, probably from Redline themselves, possibly Weber Carbs Direct, a Ratsun member has made some before and had them for sale... They run about $30.
Here is one I have (on the left):