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I'm with Scott Dahlgren, here. Forget the bolts through the firewall.

On my L-235/290, I used a hole saw to drill a couple of 3/4" holes throught
the firewall/fire-barrier in the appropriated places. Then I floxed in a
length of .750x.035 Al tubing which protruded front and back by 1/8" for
sealing with RTV.

Then I simply passed the big conductors thru the firewall and sealed the
annular space with RTV. (One must be careful to use a type of lug which
will pass thru the Al feed-through).

This saves not only weight, but two bolted joints with their inherent
resistance which are subject to corrosion and loosening over time. Should
you ever need to remove the cable, it would be a simple matter to dig out
the RTV sealant.

If you don't like the Aluminum tube method, grommets will work fine, too.