340 valvetrain help

Can't hear anything abnormal in the video but if the total timing was set at 42 it's a good possibility it's spark knock. You said you backed the timing down. Did that seem to help? What year is the 340? 70 and older models are higher compression and I usually find 34 degrees total is all they need. 71 and later are lower compression and may handle 36. I haven't had to run any more than that on any I did to get maximum power but each application can be a little different. Nevertheless I can't imagine any needing 42 degrees

Your correct about how to adjust the rockers but hydraulic lifters bleed down pretty fast when you shut the engine off so the lash you saw may not be there with the engine running. The way I adjust them is to get the cam lobe your adjusting on the heel and back the adjuster off until lash is evident then using 2 fingers rotate the pushrod on the rocker your adjusting while tightening the adjuster. As soon as the pushrod begins to drag that's the point of zero lash. You then tighten the adjuster 1/2 turn and lock it down

BTW: beautiful Duster and welcome to the board