Carburetor size

In '71 the 340 got a 800 cfm spreadbore, up from the previous 650 Avs. So the 670 is probably adequate but you could go bigger.
About 25 years ago at the suggestion of Dick Landy I put a #3310 750 vacuum secondary with 2 metering blocks on mine based on my combo. No problems, as of yesterday still on there running fine.
Landy said with the vacuum secondary it would only get more gas if it needs it. There are better choices out there now. Edelbrock makes an 800 now that dynos better on a 340 than the 3310.