76-84 vans, pickups, and medium duties are a great source for 170-prefix 800 cfm carbs. They all got 800s from the factory, regardless of 305, 350, or 454, even most of the 4.3 V6s had them. but if they were replaced by the dealer they were probably 750s so you have to check. Only way to be sure is to look down the primary throats. Starting in about 84 they went to the electronic Qjet which should be avoided for performance use. If it has an electrical connector on top above the primaries, skip it.
When I lived in L.A. I used to drive down to Jet and stop at a junkyard that was nearby. They had a whole row of trucks and vans and it was almost guaranteed that they had a few 800s I could pull.
I'm a pretty fair hand at setting up a Qjet, but I still use Jet. Well worth the money. If you live in a cool climate, make sure you tell them to keep the secondary lockout on the choke. Its a little cam that doesn't let the secondaries open until the choke is off. Without it in cold weather you get a huge lean bog if you give it too much throttle. Otherwise, I've tried to improve on several Jet carbs and couldn't. They get them spot-on