I used 3/8" spot weld cutting bits from HF. I bought two, only needed one. Go slow and steady. You will probably cut through the floor on at least one. I used a small screw driver as a wedge to lift the mount bumps so I could tell when the bit cut through it. Punch each weld with a center punch or drill bit to keep the cutter guide from walking.
IIRC, the two bits were about $5.