adjusting holley floats on shaky engine?

I am going to rebuild my Holley 750 with vacuum secondaries. I have read the sticky by StrokerScamp and whatever else I can find online about this but will be my first time. Playing around with it I tried to see where my floats are set now, but when I remove the plug, at idle my engine shakes so much I have fuel splashing out. Have a 70 Duster, 340 mild build with a MP 484 cam. Or is this much shaking not normal and I got bad engine mounts or something? Can I run the engine for a bit, shut if off, check the float level and make minor adjustments and repeat?