Hello Stan,
From the Land Rover workshop manual covering all V8 engines from 3.5 to 4.6, the standard timing sets were set so that the camshaft ran at zero degrees, neither retarded nor advanced. However, the Range Rover specialists who built my 4.6 fitted a custom camshaft along with a timing set which allows for variable camshaft timing. They set the camshaft to 2 degrees advance, for the very reason you surmised,..to increase low end torque. So I would advance your camshaft by 2 degrees Stan.
Given the changes, especially the 3.9 heads which have improved air flow characteristics and larger valves compared to the P6B heads, have you decided on which needles you will be fitting to the carburettors? The ones that were fitted originally I dare say will now be too lean.
Spark plugs too are no longer compatable, given that the early heads manufactured up until 1976 use short reach plugs, where as all later heads use long reach plugs. I can certainly recommend Champion N9YC for the task.
Ron.