Be sure and put the large area washers back on the bolts on the inside of the firewall as these will prevent you from crushing the 1/2 inch honeycomb. Just do not tighten the bolts too much when changing the relays.
You can attach a ground to the firewall surface but most folks just take a ground all the way to one of the bolts on the oil pan of the engine. This is where the main ground goes from the battery.
FYI
Roscoe