Thanks for the responses. I spent some time today looking a little more closely at this and making a few adjustments to the linkages (the butterflies were completely open before the pedal hit the floor, so I fixed that), but it didn't change the jumpiness. What seems to be happening is that the carbs are just too sensitive to inputs from the gas pedal. The slightest little movement of the gas pedal moves the butterflies too much and starts the car lurching forward.
I also adjusted the vertical linkage on the carbs per the manual to allow for sufficient free play. Wiggling the throttle shaft (at the carbs)with the rpms up does exactly what you'd think it would. The rpms go up and down.
I've got damping oil (well, ATF) in the dashpots. Maybe a thicker oil to slow down the reaction of the slides?
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'64 PVMoneypit '95 850