Wow..
Looks like a potential Darwin Award candidate.
Take the extra air flow and noise and live with it, your furnace will thank you, heck you might even wakeup alive and not on fire..
Seriously, reducing the motor speed on these furnaces is asking for trouble, they are highly compact and depend on a certain minimum amount of airflow to keep them from igniting things like the floor and walls.
So, unless you manage to dial in and nail the exact minimum airflow (which is not only on the inside but the outside combustion side) you are taking some big chances even with maintaining enough flow to keep the sail switch happy.
Perhaps you bought the wrong motor?
They DO make different motors for different BTU furnaces that run different RPMS.. I would suggest attempting to find the correct exact replacement before kludging your own work around fix.