I drilled my all at once.
One thing I did just because it seemed like a good idea at the time was alternate sides while drilling the holes. I'd drill five on one side, then five on the other working my way aft to forward. I looked back through my log and I must have done something right because my firewall fit perfectly and I didn't note any issues with any fitting of the other pieces. Do be careful of what hole you are drilling from the 704 forward. I know the front 5 or 6 are 1/8" holes and there might be a few others as well.