I am in the process of rebuilding my first mopar engine and have a question.  I am rebuilding this 1970 340 back to factory specs.  Did/does a 340 use hardened washers on the main caps?  If so, does someone know what size they are?  I appreciate anyone's input. Thanks.  

No they don't use washers on 340 main caps.
However when they sell main bolt [w/windage tray] packages from MOPAR they include hardened washers because this bolt package is universal, as in can be used with all small blocks, and since the 318 main caps are shorter and less beefier the washers are used in order to avoid bottoming out of the main bolts...
So they take up the slack for the short 318 main caps.
I have torn down a 340 once to find that someone had used them anyways in a rebuild to ,no consequence, yet no benefit. UNLESS 318 SKIP THE WASHERS.
good luck.