Just thought I'd post an update if anyone was interested or reads this in the future.
Turns out the problem was the lower intake manifold gasket. As careful as I was putting it together, and as good as I felt about it, the gasket must have slid just a tad and wasn't sealing on the lower end of the cylinder 5 and 6 intake ports.
I had a real "ah-ha" moment the last time I took the upper intake off after I had it running for a while. The intake runners for 5 and 6 were covered with oil. So it was either the intake manifold sucking oil from the lifted valley or valve seals. Since the heads were rebuilt with new valve seals, I was leaning towards the intake gasket. Pulled it apart and sure enough there was oil in between the gasket and head at those points. This also explains why I couldn't find a vacuum leak since it was internal to the lifter valley. Put it all back together with fresh gaskets and the truck runs as smooth as can be now.
Thanks to everyone for all the help and input!
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