408 Magnum New Build, Gutless of idle

I think something is out of time.

But I would not build that engine combo.
Just, IMO...
Stock R/T heads? Some valve seat work and a bowl blending since your lifting the valve as high as you are.
To light on compression
The cam (depending on the rest of the drive train and vehicle weight) is 10* to large, at a min.
The 114 installed and the installed 112 is to wide. It looses cylinder pressure. The 114 should be @ 110 or 108, the installed centerline on a 106/104.
I myself would have moved to a roller rocker.

What is the trucks weight, intended usage, gear ratio & tire size, what trans and stall converter?

If this was for towing....
On the quick, I’d swap out camshafts.