Good questions sir.

1) Alot people use the Aluminum ones as the plastic ones DO break far more often than they should. But with regard to that, consider that they ARE the weak link in the coolant flow system. Should they not be able to break and you get a pressure spike for whatever reason from the water pump (which has happened before) and the elbows don't break, you tend to blow a LIM gasket. So rather than replacing $10 worth of eblows and a $15 gallon of coolant, you get a $200 LIM gasket job to play with. Now that is the worst that will happen, but just saying, food for thought. The plastic ones are cheap and take maybe an hour of time to replace with hand tools.

2) Last time I flushed the coolant completely I only needed 2 gallons to get it back to a good mixture. I would think 3 would WAY over do it, unless you spill alot, then by all means. I would flush it all, then let the water drain out, put it all back together, add 1 gallon straight then the 50/50 as needed. As the water level left in the motor and heater core will add to the gallon of straight coolant and mix for a proper 50/50. Worked for me anyways.

3) If the tabs are broken off, and you can't get a pair of vice grips on it loosen it, you'd have to drill and extract it out with an extractor bolt. But its not really needed, just up to you. Most use the lower hose because its easiest to do and works every time.

4) I use Mobil1 5w30 Synthetic and an AcDelco filter usually, but WIX is my personal favorite, one of the best you can buy for cheap filters. Oil is all preference on these old motors, they take anything lol, but if you start with synthetic, stay with synthetic, don't jump around. Yes the cradle needs to be lowered a little, or the motor needs to be lifted a little to get the pan off to re-seal it. Its a pain to do, but worth it if you have a big leak.

5) http://www3.sympatico.ca/aepa/blowerinstall.html <--- How to with pics. Use an Anaerobic Gasket Maker type sealant to reseal the snout so you don't have leaks, and you can use GM fluid if a local dealer sells it, or Mobil1 5x30 oil otherwise. You just have to change out the oil every / every other oil change with the motor oil. It doesn't last near as long, but if you can't get it or don't want to order it, makes for a simple replacement.

Hope this helps, Good luck!