1. what is the point of different pressure radiator caps?...ive never understood this thing

2. and how reliable is the factory volt gauge in lt1 cars?

 

The higher the pressure, the hotter the coolant can be before it boils.

The volt gauge seems to be reliable, but usually the feed to the gauge and ignition switched components is compromised by a slight voltage drop. That means that your gauge may read a little lower than a reading taken at the battery (especially in the heat).