Fastest way is dig out and bury. They wont burn, not easily anyhow, itll take forever. Stump grinder is ok but chances are the dirt will erode from around whats left under ground and youll be back to hitting stumps with your bushhog and disk. Chemical stump rotter will take a long time to work. Dig and bury is most expensive but when the trackhoe leaves you can start planting right then.