The early carburetors can certainly be made to work, but more often than not, they need a lot of help to get them up to par.
In the past couple of years we've restored quite a few 1965-1967 q-jets. They are always in need of a lot of attention. Bottom plugs will leak 100 percent of the time, even the front plugs. The early 65-66 models have a piss-poor opening system for the secondary airflap. The plunger type fuel inlet deal is HOPELESS. Even with upgraded parts, they can still be very difficult to use on powerful engines.
All the negatives aside, with enough attention to the troubled areas, and complete/correct rebuilding, they can be made to be very reliable carburetors for restoration projects.
I wouldn't even consider using one for high HP levels, drag racing, etc. Best to shelve them at that point, and use a 1969 or newer Chevy divorced choke model.....Cliff