The thread Large Car may be referring to is
this one I started with pictures and everything.
If the waterpump has truly failed, then the engine will quickly overheat. And die soon thereafter. It is very, very rare that a GM waterpump will quit pumping water; the pumps are most often changed because the seal starts seeping coolant.
On the other hand, it's quite common for the heater core to get plugged up with sludge, rust and miscellaneous spooge. If both heater hoses are hot (indicating that coolant is flowing through them OK) then the next most likely problem is the air "valve" (a diverter door under the dash) is stuck. You can try resetting the position sensor by setting the temperature to all the way cold, then all the way hot. Otherwise, a little more digging is in order and you should give a more complete description of what is going on.
As for the Optispark, I did a lot of research last winter and came to the conclusion that the only Opti worth buying is the Delphi brand OEM unit. It is the only one that comes with the genuine Mitsubishi optical sensor. Every other brand is using a knock-off sensor and every other brand seems to have a high failure rate because of the sensor. At that time, the best deal on the Delphi was from Rock Auto, but at around $300 it's still twice what you can find a Chinese one selling for on eBay. I discussed my search on this forum last year and
documented the Delphi starting on page 3 of the thread.
If you replace the Opti and waterpump, you will also need a Fel-pro timing cover seal set and a means to install the waterpump shaft seal without damaging it. GM sells a special tool for $50, Buffman sometimes has a home-made version for something like $10 and some people have made do with a Sharpie marker cap. I think it's documented in that same thread, but a search of the forum should find the details.
