The only question is, how do you plan to rout and insert the truss rod if the neck is "one piece"? From the back, then fill it in?
I know there are several methods people use to construct their necks, but in our shop we attach the fingerboard to the neck shaft which effectively 'covers' the truss rod channel. Of course, by using this method then neck is no longer one piece. Hence, I am curious about your method. Because we don't make one piece necks here (typically) I reserved my comment strictly to the truss rod and related dimensions.
Anyway, take an average neck thickness of about 7/8" or in the .800" range around the first fret. Typically we make our fingerboards slightly thick or a heavy 1/4" - 5/16" to start. So let's review the math:
LMI style dual action truss rod for reference
1/4" fingerboard + 5/8" thick neck shaft (I'm rounding for this conversation)
t-rod channel - 3/8" deep. (don't forget, you may need to go a little deeper than that around the adjusting nut to give it clearance)
That leaves roughly about 1/4" under the truss rod (between the active rod and your hand)
You're using a 1-1/2" thick piece? You have plenty of thickness then.
Hope this helps
-Doug