"The truck ran like a top before the swap"
This would be reason enough to swap the 2-bbl back on it.
You more than likely have a serious vacuum leak. If you can't find it by spraying a combustible vapor around the intake and carb base, then it is probably sucking crankcase vapors into the intake ports from the interior side of the intake manifold.
When we change manifolds, we all assume that everything is machined to match up perfectly. Nothing could be further from the truth. If the intake and head surfaces are parallel in both planes, it is by the sheerest of coincidences. Please read this response to another member:
Lean condition possibly due to the inlet ports sucking crankcase gases. If the intake manifold/head surfaces are not perfectly square and parallel or if the intake gaskets are not properly fitted to cover the ports on the interior side of the manifold/head interface, this can happen. Ask me how I know.
You have covered everything else that could be causing this, so that is my best guess. I suspect that if you remove the intake, you'll see where the gasket has not been clamped on the interior side.
Measure the thickness of a new gasket. Get flat washers or shims that will measure that thickness. With the manifold off and the mating surface on the cylinder heads de-greased, put a dab of RTV on them and stick them on each corner bolt hole on the cylinder head. Let the RTV set up. Stuff paper towels into the ports to keep debris out. Make up 16 pea-sized balls of modeling clay. De-grease the intake manifold at the ports. Place the balls of clay on the top and bottom of each port, squishing them down well so they will stay in place. You want them to be thicker than the shims/washers that are RTV'd to the heads. With your fingers, coat a little oil on the heads where the clay will meet the head to keep it from sticking to the head. Now carefully place the manifold into place on the heads and use bolts on the four corners to just snug the manifold down until you feel resistance against the shims/washers. Remove the manifold carefully and mesure the thickness of the clay at all 16 positions with the depth function end of your 6" dial caliper. You'll know pretty quickly if the manifold/head interface is square. Record the measurements on the manifold with a permanent marker like a Sharpie. The widest measurement will be the standard to which you will want your machinist to cut the other positions on the manifold to make it square with the heads, thusly sealing the motor up.
I've explained this procedure to others who then have looked at me like I'm from another planet. Am I the only guy who does this?