How much "shake" at idle is acceptable?

No I don't. I don't think my shop has one either but I'll check. Yes, the engine only shakes at idle. Runs pretty smooth above 1100 rpms.
Like the fellow on this thread. He's disassembling his engine to find a shake he (or members here) think might be mechanical or a imbalance. It will be fun to see if he ever discovers what the issue was and at what cost. 340 Engine Shake Note: his problem was a cam and lifter issue.