I’ve only been able to find these and it’s only three. Does anyone know the actual size of the heater housing nuts that hold it onto the firewall and if it’s just a bit like this. Are there washers? One of these seems to fit but has a bit of trouble halfway down the threaded stud.

 

I tried to look up those nuts but can't seem to find them in any of my parts catalogs.
I call those flange nuts and just guessing here but thinking that any nut and washer in the appropriate size should work.
Dealing with old hardware in general I invested in one of these.

https://www.tooldiscounter.com/produ...yABEgIAgvD_BwE

Use cutting oil when you using the tool.
Comes in handy too cause I have added a lot of options to my '00 that she did not come with.
Many times the used hardware can be replace with stuff from the hardware store.
But here and there the hardware is very specific and there is no substitute.
That is when this tool comes in handy.