How to get rid of understeer in a 1970 340

I would start by lowering the rear to get the roll center lower. This will actually increase understeer, but I agree with 72bluenblu that this needs to be lower; with the rear in the air, there will be a lot of transitional oversteer as you dive into a corner, and it will be hard to make a good transition to the desired steady state over/understeer. And he makes a good observation on the low front end...do you ever bottom out the front suspension on bumps or strongly undulating raads?

The added rear sway bar should help your understeer. I cannot tell how much the HD rear leafs are helping, but would not assume they are stiff enough to get the right rear roll stiffness. (I just don't know the spring rates).

Give us better info on what shocks you have front and rear; there are a huge variety of gas shocks. These are important in transitional steering attitudes; thewring match F to R will cause the car to do odd things when diving into a corner.