You probably won't find a Harper they haven't been made in a while.
The three-1s are more common than the three-2s - but they are around. In fact there is one on ebay right now:
ebay Harper 3X2 Intake 
And they are not cheap. To Beater's point multi-carb setups improved fuel distribution in log manifolds. An adapter mounted in the center of a 4BBL manifold doesn't help in this regard. Adding another plenum above the entry into the main-manifold will likely result in to-much plenum volume for the cylinder displacement.
regards,
stock49
