I left mine as is with the existing plate in the back. There were versions that came with blocks, or you could add them. One thing the AFB's and brethren have over the Holley and its cousins. There are NO passages for junk to get stuck in, unlike Holley. If you get crud in a metering block, it's a battle to get out. Ultrasonic tanks are very helpful on this. You can buy metering blocks new from Holley, but only for certain LIST numbers. Probably fat chance on an older 3310. The best thing you can do if running a Holley, provided it's not already blessed with a bad block, is be very diligent on fuel filters. Rust will get by every filter on the market and get embedded in the blocks. However...rust won't get by a magnet. On the wagon, I have a Mopar trans pan magnet on the filter inlet. Metal WIX. The magnet is about 2" od with like a 1" id hole in the center.
Back in the late 70's, I was running a six pack on my 73 340 Duster. Bought the carbs new. The primary was a problem child from the get. I lost count of how many times I pulled the primary carb off to blow out the block. Put it back together, would run fine for about 10 minutes, then start acting up again. A few years later, my brother put the block into an ultrasonic unit. He said, I was working at the time, that the amount of junk that came out of that block was mind boggling. Worked fine for about an hour, then back to running bad. Later on I ordered up a new block and no more problems.