They are the same carb with a few changes like was already mentioned, they have quick change secondary springs and a secondary metering block, very common for people to add them on a 3310 so just use the avenger that already has the goodies on it, or better yet get a thermo-quad.

PS the only holley carb I ever had that ran good was a 770 street avenger and it worked great on my 318, it never got close to the miledge as the thermo-quad but it ran good.