1999 Cadillac Deville Replacing the A/C Condenser

Yes, you need to remove the radiator. 300 psi hi side is not too unrealistic since you are stationary, and also picking up heat from the radiator. How do the cooling fans act, are both of them running. That will cause your pressure to rise as well, and also what the ambient(outside) air temperature is. Try a box fan or other device in front to simulate ram air, and see what the pressure does. What led to the parts replacement, was it compressor failure? How contaminated was the original orifice tube. Often times the need to flush is overrated unless something other than the proper refrigerant and oil was introduced to the system.