69 barracuda dash to firewall bolts

I was thinking 1/4-20 bolts. The threaded neck of the 'U' nuts wasn't much taller than the head of the bolt. I decided so long as the bolt doesn't come out of the neck, all good. Yeah the heat case and the left fresh air think need to be removed. So if you find you need about 5/8 bolt length, whittle down the 3/4 length rather than search the world over for exact.
The only fastener in a slotted hole is the firewall side of that left vent.