A power slip extends a joint from tubing disposed in casing at a well so that a casing head can install on the casing and the tubing can be packed-off from the casing. The joint attaches to an inner sleeve having collet members that fit onto the inner tubing so that the joint extends thereabove. An outer sleeve and interlocking collet assemble together and install onto the inner sleeve. The collet threads onto the inner sleeve and forces the outer sleeve downward onto the inner sleeve. Being forced, the outer sleeve causes the collet members to engage around the inner tubing. The casing head then installs on the casing with the joint extending up through the head's bowl. An H-plate installs in the open bowl around the extended joint, and another pack-off installs on the joint to pack it off from a spool attaching to the casing head.