ATMEGA 328P Standalone on 3.3v controlling a servo
do I have to convert the PWM signal's peak amplitude to 5V for it to work?


There is insufficient information to determine that.  It may, or it may not. You'll have to try it and see.


  since I will be using an 8 MHz external clock, do I have to change anything in my Arduino sketch regarding the PWM signal frequency


If the board you select is 8MHz then no, you don't need to change anything. The fact you have selected the right frequency for the board means that all the timing calculations the Arduino API does will be correct.