I used a step drill to drill the hole in the firewall, widening the hole to 3/8", then used a step drill to finish it through the engine mount bracket. I gave up halfway through, though, and went to Ace Hardware to buy a new step drill and 3/8 bit and finished it off. Then I did the other side and deburred the holes as bestI could.
I installed the bolts through the engine mount bracket and then attached the engine mount. It fit fine.
I found the castle nuts but couldn't find the washers called out, so I used some old 610 washers from the previous build for a temporary installation to get the lay of the land. Then added the castle nuts.
I need to get a clarification as to whether these are torqued down for tension and, if so, what the callout is because my AC 43.13 does not indicate a torque for castle nuts.
That firewall has a lot of flex in it still.