Hey guys working on my buddy’s 340 dart he just got, pretty much stock 68 340 compression heads valve train,etc only thing done on the rebuild is they used a size bigger Mopar purple camshaft (as were told)
All right, first off you need to post some pics of your rockers. Stock '68 340s did not have adjustable rockers, so we need to know what you've actually got.
At this point I'm going to guess stock '64-'67 273 mechanical rockers. You didn't say what the replacement cam is- hydraulic or solid? If hydraulic, the stock 273 pushrods cannot be used and custom length rods need to be checked for and ordered. If it's solid, the pushrods MAY be able to be used, depending on the brand of lifter used- again, checking and ordering the correct ones if necessary.
The other issue with these rockers is that the adjusters are an interference thread, and after 60 years of use they lose the "tightness" in the threads and no longer maintain their adjustment.
The common fix is to chase the threads in the arms and spot face the top of the arm; then install a jam nut. Crane used to offer upgraded adjusters but since being bought out by Comp Cams they have not been available... Alternately, some people will just give the rocker bottoms a couple taps with a hammer (to close up the threads) and re-install the adjusters.