I added more heat registers in my basement and wonder if I can up the size of the pulley to increase volume through the registers without damaging anything. Currently have a 1/4hp to run a 100,000k furnace to heat main floor (1200 sq. Ft along with another 1200 sq.ft in the basement)

This might increase the volume of air, but its going to be cooler air, maybe to cool.
You have to have a higher BTU (more firepower) furnace to get more heated air

Thank you, I think I should try it.