What carb to go with ?

[chopttop]

Changed oil, fuel filter, plugs, wires, cap/rotor (HEI) on the '71 C 30 cab/chassis camper I bought last fall, Idles and sounds great for a minute or two and then it seems like it starts to load up and i have to rev it up to clear it up and then it starts all over again. It is a 350 w/a quadrajet, needs replacing anyway, support broke off on one side of plunger activating arm so it,will not stay centered on the top of the plunger? unless I use a small vise grip to hold it in place. Also found out either the engine isn't original, imagine that, or they put older heads on it, sounds good when idleing, have only driven it , 2 miles , home, when I bought it, very slowly. Which carb and what size should I be looking for. Thanks