Happy New Year!
Definitely: Computer and wire harness to make the TPI system go - also will need the fuel pump/level sender - may need to extend the pickup. Front sway bar, steering gear (need to exchange the Pitman arm - with luck the steering gear in your Century has the four-bolt top), mirrors, headlights, rims and lug nuts (will need one pair of spacers), front rotors and calipers, lower rear control arms, complete rear disc brake setup plus cables, proportioning valve for four-disc swap, limited-slip differential plus ring/pinion (if not an Aussie 9-bolt).
Consider: Wiper motor and wire harness (mods required), console (brake handle will now be on the wrong side), seats and seat belts (may be swappable), tilt column, donut spare tire, entire rear axle - you could get wild and try to fab a GNX-like three-link with Panhard bar - keep the torque arm and trans mount plus panhard bar with chassis bracket], exhaust system could be modified if not too rotten, A/C system may be swappable - pull the whole thing out as a single piece if there is freon still in it.