Also, do not think that you are tied to using 16MHz with 5v and/or 8MHz at 3.3v.
All the OEM “ATmega328” hardware (bar the Arduino emontx-shield) are running at 3.3v with an external 16MHz xtal.
Even the pro-mini’s CAN be 3.3v 16MHz (the’re the ones I use generally, although it’s been a while since I have) despite most info telling you otherwise. You need to hunt them out and be sure of what you are buying (vs what you get supplied) when looking at auction sites etc. I haven’t looked recently, but even the Arduino IDE used to only list 3.3v/8MHz or 5v/16MHz versions, if that is still the case you can add a custom board entry (that’s what I did) or just use the 5v/16MHz option without a problem despite it being 3.3v, the IDE only gets involved in the clock speed, it is unaware and uninterested in the voltage.