HEAT...but you'd better have the rugs and seats out,or else risk burning the truck up...
GM used that red locktite on those bolts,95% of the time you'll bust a bunch of torx bits off trying to unscrew them,or in the case of the shouldered hex head bolts on older trucks,they usually tear out the floor around the weld nut,or shear off in the nut,if you try removing one without heating it up good first..