How much "shake" at idle is acceptable?

Well therein probably lies the answer.
The stock 340 made peak hp & tq @ 5000 & 3200 rpm.
To make peak hp 600 rpm higher would require a longer duration cam, & they often come with a lumpy/rough idle [ included in the price! ].
So you are probably chasing a problem that is 'normal' for the parts combination.