72 Duster help

Hi, I'm just across the border from ya..
Easiest answer; if it's a stock 318 it will be a 2-barrel, unless someone's slapped a quad on it. Also, the 340's had different exhaust manifolds. Look a some photos on this forum or Google and get an eye for a stock 340 and you should be able to ID one by the subtle differences. Like this one;

https://www.google.com/search?q=340....com%2F1970-1976-plymouth-duster6.htm;400;285

You can see the HiPo manifold on the driver's side.

As for the 8-3/4, as was mentioned, there is no rear cover...it's round and smooth and a 'front-loader'.
Here is a link to the 8-3/4 ID factors....

http://members.tripod.com/~Mojo_Page/chry875.htm

PM me if you have any questions. Happy to help.

BTW, even if it is a 318, that's not a bad thing. They can be made to walk & talk with almost as much authority as the 340, and you have a first-class engine shop right down the road from you.