Tyler,
I bought a 2X4 from Randy like yours. I put a small blower on mine and it seemed to work very well during a test boil. I believe the 4.5 inch gap in your tunnel is too much. I have about 2 inches for mine. A cap at the top of my 8' stack was too restrictive so after removing it my stack temp went up to 900F. I leave the cap on when not boiling.
More tweaking probably will be necessary once I start boiling, but for now everything seems like it will work pretty good.
Peter
1991-1993 traded sap for syrup, 30 taps
1994 small flat pan outdoors 30 taps
1995 barrel stove, small pan outdoors 30 taps
1996-2011 homemade 2 X 4, sugarhouse, 50 taps
2012 new 2 X 4, 60 taps