I'm replacing heater core, what else since it's apart

[mpj76]

If you haven't already noticed, the instrument cluster circuitry is a plastic film with wiring embedded in it. It is very fragile at the connection point. If you accidentally tear one of the connections, you will, of course, lose some gauges/lights. If you try driving around without the dash torn apart and the cluster disconnected, your alternator will NOT charge the battery. The voltage source which excites the alternator at startup shares the same circuit as the cluster. One way to excite it manually is to blip the throttle to redline redline shortly after startup.

Haynes manual requires disconnecting AC lines. Article in MM&FF (sorry don't remember which date) does the job without it. Good luck. Keep all screws in carefully labeled baggies or you'll end up with dash squeeks.