New firewall pad installation 77 T/A ?
 

Would you use the old rubber retainers or the plastic ones that come with a new one?
My old rubber ones are still good.77 trans am.

Old ones if good. The repo ones flex and bend too much and are hard to get thru the hole

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yes,old are good,not hard after 42 years!

on a related note, was there fibreglass(?) sections inserted into the ends of the firewall?

I pulled out a tattered chunk of fibreglass from mine;
the day after tossing it, I thought to myself maybe it was put there from the factory, so I pulled it out of the garbage, and pieced it together for a picture.

not on this car.

maybe someone else can tell me what this is about(?)

https://forums.maxperformanceinc.com...1&d=1563406864

I think that is what is glued to the underside of the dash top surface. They make a replacement

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I have rust in the usual spot of the bottom corners of the firewall - visable when fenders are removed - infront of, and below the hings mounts;
I pulled this out of the little hole there.
So unless I have grossly misunderstood you, I don't know if "dash" is applicable...

Thanks for offering your suggestion though.

I'll keep this chalked up as a mystery...