There is a fairly new flexible gas hose now available for direct burial.
Its yellow in color and comes in various diameters. I once ran a gas
line from my meter to a small addition where i used a 50,000 btu input
heater ; i ran 3/4" i.d. gas line about 125 feet and it worked fine
without increasing gas pressure. Id try running a 3/4" gas line and
have someone check the furnace manifold pressure once its installed and
running. If it isnt approx. 3.5 " w.c. , then the Gas Co. can bump up
the incoming gas pressure.