What might happen if that venting will allow the unit to move more air. That may well overload the motor and burn it out. You can test this with an amp meter if you want to stay with that unit. Read the amps and compare the reading the FLA number on the motor plate. In all honesty, I would be surprised if your happy with the performance of that unit. The reducer for the exhaust can be as simple as a piece of plywood cut to fit the DC outlet, then put a 4" (+/-) hole in it to accept a PVC union, secure it with screws and seal it, then pipe your pipe into it.