Who can I ask to install a 30 amp rated range on a 30 amp circuit (not as easy as it sounds)
The appliance installers are not electricians.  They are refusing to install only because it's not what they're used to, and they don't know why it's different.    They gave you bad instructions: You cannot install a 50A range socket on this circuit.  It is illegal to do that because socket sizes must match the circuit/breaker size.  Even if the wiring in the walls really was 8 or 6 AWG, the breaker is 30A so the socket must be as well.  

Given that the manufacturer's instructions say to use a 30A cord, and plug into the socket, that is the Word From On High.  If it specifically calls out "do not use a 40/50A range cord", that too is gospel.  It is mandatory to obey the instructions and labeling of an appliance.  

The NEMA 14 type connector is definitely the right thing.  Don't let them talk you into installing the old 3-prong NEMA 10.