There are more too it then just adding two pulleys and a belt, if you want those speeds you will have to balance the motor use quality Bearings in the motor and cut the shaft down on the motor, if you are never going to change it back
The Motor shaft Diameter is 1.1785" there are some with different shaft sizes, so you will have to measure the one you have, there are other Bearings that are under the front drive, that can create noise, as for the front shaft, I have never used the original parts, and custom made new shafts, for the machine builds I used to do, you may be better off replacing the Motor with a Baldor wash down motor which are rated to 6,000 RPM, or similar motor from other manufactures
The front Bull Gear Shaft is 1.378" but your could be a different size also
You would be better off to get a speed doubler, or make one to mount on your spindle, so you can run those small cutters, here is one for an idea, they can be more compact than this, there was one available for Bridgeport Mills https://www.tormach.com/store/index....ef=speeder_kit