Keep in mind the basement vent components don't need to be 100% airtight for the vent to be effective. You want a free flowing chimney and enough exhaust gas temp in order to ensure that there's a vacuum at the chimney inlet. That way any leaks in the vent piping pull in room air rather than leak out exhaust air.
Adjust your draft control damper properly and make sure the room the boiler is in has provisions to supply combustion air. And make sure the fan you have set up doesn't create low pressure in the basement.