Troubleshooting the vibration got sidetracked by having to replace the fuel pump - converted to electric. I did check the water pump and generator, both seem to be OK. While I was in there I replaced the 20 yo belt.
The motor mounts are all snug. I'm starting to think it might be misfire related. While idling, It did pull each plug wire one at a time. At first the back two had less effect than the front two. I tweaked the idle screw on the back carb and now pulling any wire has the same effect. When I have more time, I need to go through the whole carb balance procedure again.
I want to repeat the spark plug wire test per Charlie's advice at speed. While driving around tonight, it seemed like the vibration got worse when the engine was completely warmed up with the water temp at about 180 degrees.
I still need to dig into checking the 1/4 marks - I'll have to get the manual out to see what they are.
This is not a huge problem, the car is very drivable. I'm just trying to learn about what's normal and not normal for a 50's technology engine. I wish I had another sprite to drive for comparison.