Lack of a functioning heat riser means the manifold is dead cold way too long, requiring rich mixture when it's not necessarily what the engine wants.
Closing your manual choke cable half-way isn't where it should be for dead cold - it's still too lean.
The choke blade is normally fully closed when cranking, but once it starts you should open it 5/32" measured between the edge of the choke blade and the throat. Use the stem of a 5/32" drill for this, and mark the cable position in the cab with a piece of tape or a stop.
The Model BV diagram shows where http://www.chicagocarburetor.com/pdf/Instructions-Rochester-B-BC-BV.pdf
The truck can and should be driven in that position at light throttle.
Open the choke with the cable gradually as the manifold heats up. You'll know it's still too lean when it hesitates as you roll the throttle on.