I've never seen a plug for that. I just welded steel over the holes when I completed the smooth firewall similar to what Steve did on Penny.
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Steve Spoor
69 Rally Sport with Quadra-Link, Wayne Due C-4 Subframe, T-56, 420 SB2.2
67 Chevelle, Art Morrison Chassis, 416 LS3, T-56 (under construction)
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