When setting the specifications for all of the components going into an engine build, I cannot say that any one item is more important than any other, as they all need to work together. However I do believe that the camshaft specifications has more impact on the ‘personality’ of the final motor than any other single item.
If the goal of your build is low RPM torque, the right cam is what is going to make that happen.
You want a small cam, around 200 degrees of duration or smaller, medium lift. With an upper RPM cap of 4500 rpm, I would stay with stock sized valves, do some minor bowl clean up and invest in a good multi angle valve job.
To maximize performance you need to maximize compression and the ignition curve.
For the compression: get knowledgeable on Static and Dynamic compression.
For the ignition curve: a heavy vehicle usually won’t take a lot of initial timing, but with the right springs you will be able to bring the mechanical advance in as the vehicle starts rolling.
Remember, you are working with a small displacement engine and a large vehicle, be prepared to be underwhelmed.
In summary, it is understandable that you want to maximize low end torque, a good flowing cylinder head can help, but it is not at the top of the short list of things that make low rpm torque, which are:
1) large displacement, which you do not have
2) camshaft profile-one with an early closing intake valve to build dynamic compression
3) compression, static and dynamic
4) ignition advance, mainly when the mechanical advance comes in and how it ramps.