1) If you buy a new core, the OEM is better quality than the aftermarkets available at my local parts places. It's more expensive, but with proper coolant flushes will last you more than the next 20yrs.
2) When doing the heater core job, here are a few hints. A) There are two 17mm nuts holding the steering column up to the cross brace behind the dash. Telescope the wheel out as far as it'll go, move the driver's seat forward, remove the nuts and let the seat support the steering wheel. No need to remove the actual wheel. B) Unplug the dash harness at either side. LEAVE IT ATTACHED TO THE DASHBOARD. Those two small tips alone will make the job SO much easier for you.
3) You CAN wriggle the heater core box (white duct work) out by loosening the AC evaporator box. You do NOT have to drain the AC system.
4) Be sure to properly secure the heater core pipes to the white box, there are extension pipes on the 'core that use nothing but rubber o-rings to seal it. If you tweak them, it WILL leak.
5) If the extension pipes that go through the firewall grommets are bent in any way, replace them. They're cheap enough and if you're doing the job, just do it right. It'll make your life so much easier. Buy new constant tension clamps for them. Most of these pipes get crushed because people crank down on them with worm gear clamps and destroy them.
6) When removing the dash, pull it out the passenger side of the car. Be careful not to break any clips that secure it just below the windshield- it'll rattle if you break them and don't replace them. When you've got the dash loose, pull straight back to pop it out. Easiest if you remove the shifter, though not necessary.
That's all I can think of off the top of my head.
