fast idle will cause engine RUN on..

there are several things that can cause FAST IDLE on that year.. i still don't know exactly which model you have.

is the electric choke opening fully.. with the engine warmed up after about two minutes of operation???

is your carb equipped with a secondary choke pull off.. rear passenger side of the carb.. that gets vacuum from the black valve in the side of the air cleaner. with no vacuum applied or the hoses hooked up WRONG it won't pull the carb off fast idle and you will have issues.. if the diaphragm inside that is leaking. you will also have a fast idle stuck on issue.. warning a some of those have a calibrated vacuum leak built in so the vacuum to move the diaphragm needs to be more than the calibrated vacuum leak..

can you get the stamping number off the drivers side of the carb.. it will be STAMPED IN behind the throttle lever.. sideways.. 170XXXXX with a second row of numbers below it.. this is STAMPED in not cast in.. there are 7 different carb versions on that with a 350 engine.. so a carb number off the carb main body.. is needed.. California emissions.. Federal emissions. Canada emissions?? it says that on the label under the hood..

you could just have a vacuum hose leak... or even a bad power brake booster.. perhaps an incorrect sized PCV valve..

one van owner had a vacuum leak in the hose to the big plastic vacuum reservoir that is mounted to the inside of the hood.. if the hood was open the heater control system worked.. close the hood and the heater controls did not work.. please list which motor.. perhaps the 8th digit of the vin number.. if you can.. take a photo and post it of the emission label under the hood with the vacuum diagram on it.. if not.. cover the VIN number and take a picture of the build label with the rows of 3 digit letter codes.. i can figure it out from that. it is probably under the hood.. could be in a door jam or in the glove box ..