I'm sure you're right. I was looking at a couple 850 blower carbs for my engine when this came up. It was significantly cheaper than the carbs and I needed a bunch of other stuff to convert from a Bugcatcher FI to carbs. It is a race engine but I may never race it. I have another engine I'm building which will be a 360 street engine with a Procharger which is planned to go in a Notchback I am going to restore. Who knows by the time I get all this done there's no telling what will end up where. This is the car which I was planning on putting the webers on.

Look around for a local Weber guy and go from there. If you can find an old timer who knows Weber carbs he'll be able to help you get it running. I'm sure the carbs all need to come apart and be gone thru and it would be good if he had some little engine to test each one on before you try to fire up your blown engine with them. Good luck, I have no idea how to find a Weber guy except word of mouth with some foreign car guys.

IMHO a Weber expert in NA engines isn't going to be much help without blower experience.
Webers are designed to be run as IR: one bbl. feeds one intake port with almost no plenum volume separating the intake valve from the venturi. This is why the CFM of one bbl. is not 1/8 of the total displacement air flow.
I've never seen 4 X 2 Webers on a blower.
The vacuum signal from the GMC is completely unlike any NA engine. Important: a small blower, or one turning too slowly, will still have vacuum at light throttle. A large blower will have boost very quickly, so the power system (where that is in a Weber?) will come on at different RPM and throttle positions depending on how large vs. how fast it's driven.

I may have something on my site here: http://victorylibrary.com/mopar/blower/blower-carb6.htm

Yes, the signal that the Webers would get from a blower are not going to be the same as they would in an IR application. I think the Webers will work on a blower in a street rod application but I doubt they'll work well enough to go racing. A person should be able to figure it out with a few phone calls to the right folks. OP could start by calling these guys who sell a quad IDF for blowers: https://jiminglese.com/supercharger-carburetion