I have used the Fiat Weber on a stock A-12. It ran pretty well too. I wish I had a picture of it, I used a hot pink Frisbee flying disc as the air filter cover. My boss hated that! He was glad when that car became the Chickenhawk.
I'm not sure of the size but I thought it was smaller, like a 26/32 or something like that. Of course it is pooible that Fiat used several different sizes. The other thing to take into account is the primary and secondary are reversed in that carb so the thing works backward from a stock carb or a 32/36 Weber.