Is it an after market programable Tach? If it is then you need to tell the Tach how many cylinders you have. The after market tachs are made to fit 4, 6 and 8 cylinders typically. If it reads 2,000 when the ignition is turned on it may be set for a 4 cylinder, but then again the readings would be completely off when the car is running.
I would check the ground to eliminate noise from for the temp guage. Do you hav ethe proper grounds inplace from the frame to body and body to block etc? Quick check it i with gatro clips to see if it helps and move the guage ground if it helps.