How to choose a camshaft

Increasing duration will tend to shift the power and torque curves up in RPMs.  Longer durations lend themselves to higher RPM operation.  Why?  At high RPMs, the amount of time the valve spends open is smaller than at lower RPMs.  Keeping the valves open longer allows the cylinders to fill with more air and fuel.  Since the valve is open for considerably longer than the intake stroke, it does tend to reduce power and torque in the lower RPMs.  At lower RPMs its open too long and some of the good stuff you just sucked in there gets pushed back out because the valve is open longer than optimal for low-RPM operation.  Another important factor to remember is that larger engines tend to "tame down" a cam's duration.  The same duration cam in a small displacement engine will have a higher peak RPM than if you installed it in a larger displacement engine.  For example, if a cam provides a 6500 RPM peak hp in a 305 Chevy, the same cam might peak its HP at 5500 in a 400 chevy.  Here is a comparison between two engines.  The only thing I changed about these two simulations is the duration of the camshaft.  Notice that the engine with the larger cam makes more power, but you would have to rev it 1000 rpms faster to get it.  Notice also the huge loss of torque down low.  This is an extreme example just for comparison.

Lobe separaration for a given duration will alter a few different things.  Primarily it changes the amount of overlap.  Narrower LSAs will increase overlap.  This has a tendency to reduce engine output at lower RPMs and increase engine output at higher RPMs.  Narrower LSAs tend to make more peak power but a little less average power.  Wider LSAs tend to make less peak power, but a broader powerband.  Changing the LSA also changes the valve timing events; opening the exhaust valve sooner and closing the intake valve later, both of which affect how the engine ingests air.

To summarize, the whole system has to match; carb, intake, head flow, exhaust, cam, torque converter stall speed, rear axle ratio, tire size, transmission ratios, and vehicle weight.  Some of those things are already decided for you within a small range, like vehicle weight and transmission ratios, while others are easily altered like rear axle ratios and tire size.  Choosing a cam with this knowledge in your brain might make it a bit easier to understand the reasons why a professional might recommend a certain cam, and it might help you make wiser decisions about your cams in the end.  Either way, the right cam choice can make the difference between a well-sorted drivetrain and a clumsy, finnicky engine that won't put a smile on your face.