Your boost gauge is correct reading as it does.. -20 is in vacuum (car is not under boost) at idle, where it should be. Peak boost should be around 15psi in 3rd gear.
As for the EGT gauge, if you want the most accurate reading, you want to tap the exhaust collector on the passenger side of the car near the front (next to the oil filter). Simply remove it, drill it right on the ridge that sticks out, tap it with appropriate pipe thread size (should be about 1/8NPT), and insert the temp probe. All hardware except the tap should have come with the gauge.
As for the oil pressure gauge, just purchase the sandwich adapter for about $50, it's the easiest way.
Under constant pressure