Years ago I pulled a set (NOT A DOWN DRAFT)of dcoe webers of a wrecked 140 with a B20 that had been running Nitrous. They had a tank in the rear trunk, a line running up to under the drivers seat, manual needle valve there to a split line. Each line ran to the port on top of the MISAB dcoe manifold. I have not clue how well it really worked as the car was totaled, the head and block was clearly cracked. I bought the carbs and manifolds off the car and walked away from the rest of it.

But as the other's said, a down draft weber is not a performance set-up. For most folks downdraft webers are a carb set up that required less tunning than SU's.