Remember that the water pump requires a special socket: the entire unit - pulley, seals, bearings, impeller - is a unitized capsule which unlocks by turning clockwise (remember the tang orientation and amount of engagement) and installs by turning counterclockwise to lock. When I say "turn" I mean with force - some of these puppies are a bear to remove. You'll need the special socket with a retainer strap to hold it properly, or a strong assistant to help keep the socket engaged.
The coolant level is indicated by a rather indistinct arrow molded into the side of the surge tank. About halfway up in the tank, COLD, is the proper level. Too much coolant and it tends to blow out the cap relief as it expands. The Northstar has a large coolant capacity so there's considerable expansion and contraction during heating and cooling.
BEFORE you remove the water pump, do a quick check: remove the surge tank cap and have someone start the car. (There's a 3/8" line which runs from near the thermostat housing to the nipple at the surge tank top side. That's a purge line which constantly passes air bubbles (if any) from the main system to the surge tank, preventing pump cavitation.) Does coolant squirt into the surge tank ? If not, while you have the system open, remove and clean the purge line and the nipple/bolt at the water pump area. That nipple tends to clog easily, defeating the purge system.