I bought the cones ... wouldn't longer springs lift the body up, so you'd need longer shocks too ??
Yes and no.
Remember the suspension is a system and not a single component. So to get it to work to it's best you need the right mix of components.
As for body, well with ladder chassis vehicles it is better if you think of it as 3 sections.
1. Body
2. Chassis
3. Axles.
As you can lift the body away from the chassis in a different kind of lift.
Lift springs will lift the chassis away from the axles, it just happens the body is attached to the chassis.
As for the longer spring. Well you get different kinds of longer spring. Some are a higher spring rate and will lift the vehicle at rest. This doesn't affect the total travel you have available, but means the shocks are already more extended. So when you require a wheel to droop, they have less available downward travel.
HD springs may also require more weight to full compress. So while the amount of upward travel is unchanged, you may not be able to utilise it.
And depending on the spring design, it maybe become coil bound before you fully compress the shock so could lead to less upward travel being available.
There are longer softer springs, these shouldn't really lift the vehicle. But will remain seated for longer, as their uncompressed length is greater.