yeah its the same duct work I think its the small ducts thats making the pressure... I have one shut offSounds like it is surely enough total sq inches to accomadate the furnace...surprised that your SP is that high. Are there dampers in the duct system that are partially closed?
Is this the same duct system that the Clayton ran to? If it was big enough to distribute the heat from that monster it should do the job for the VF...