You can get the correct 5/16 fitting from
Carburetor Parts & Kits for Carter, Ford Motorcraft, Holley & Rochester Carburetors
5/8"-20 x 1/2"-20 Fuel Inlet Fitting, OEM # 7000262
Our Part # [2218C]
Quantity 1+ 10+
Price for each $12.95 $10.36
Your savings - 20%
Only place I found the correct fitting for my 383 using the AVS 800 and the stock fuel line from pump to carb.