[prkaye]

Are there any limitations on the use of the thinner AN460-xxxL washers in structural areas? Is it ok to use one on the AN6 bolts that go through the firewall to engine mount, to get the nut at just the right depth?

 

[rvbuilder2002]

That is actually the main reason that both thin and thick washers exist...to make adjustments for proper installation of nuts.
The standard guideline is that up to a maximum of three washers is allowed to be used on a fastener The can be mixed between the bolt head end and the nut end).
If you need more than three, you should change to the next shorter bolt/fastener.
If you need to turn the bolt instead of the nut during installation (to be avoided but sometimes there is no other way) it is a good practice to have at least a thin washer under the bolt head.