Sad that someone welded the pulley. These come apart from the top after the nut and pulley are removed. Cutting that weld will likely destroy the top of the shaft along with the pulley. You might get lucky by drilling 1/8" holes partially through the weld and break it free, but it will have to be rewelded once the bearings are replaced. That is hours of work.
I suggest you replace the spindle along with the pulley and new hardware. Check on eBay for an 8-TEN brand deck rebuild kit, it will come with everything you need. On occasion a single spindle can be found with all of the hardware and top pulley w/nut.