i'm doing a similar setup on a small block z with four 40mm dels. i got my carbs direct from italy from a guy who sells them on ebay.Cheap! ,matched and nice. carb mods for the blow thru are as follows. foam or filled brass floats. sealed or o-ringed throttle shafts. a static fuel pressure of four to five lbs. plus one lb. of fuel pressure per lb.of boost pressure, this is where the boost referenced fuel reg. comes into play. if for example you are putting out 15 lbs. boost you will need 19 to 20 lbs. of fuel for engine supply and to overcome the 15 lbs. pressure that is in the float bowl. in a blow thru set up, if the float bowl is not pressurized as well, the fuel will not be able to enter the fuel stream.
good luck