so you trust a drywall person to know the codes for venting an appliance and when you get information from the HVAC guy you question that??!!
single wall vent pipe, one thin layer of metal, needs 6" ( if memory serves me ) of clearance to combustibles where as B-vent only requires 1" clearance.
If your drywall guy did the hole for this, I would question the 29 years experience!!
most furnaces have specifics on how close things can be printed on the information label inside the upper door of the unit as well as in the instruction manual.
there is a minimum requirement for combustion air that needs to be considered as well. call that HVAC guy back if you cannot find the information, seems he has your best interest in mind!!