Occasionally with 'glass you will have a crack. A 6" crack is substantial. There are several reasons, but there are two that I can think of as being the most obvious. First are the materials used and technique. If the firewall is manufactured with chopper gun short strands, there is little inherent strength unless there is some sort of backing of 'glass or Kevlar cloth, steel or even plywood. Then of course, being a roadster, and secondly frame twist/body twist will cause some failures as there isn't a roof to hold things solid. My steel '31 is built similarly to a '32 and I can assure you that the cowl/firewall area flops unless it is solidly bolted to the boxed chassis.

What to do - check for twist by jacking a corner and measure as best possible how much the frame and body twist (or spread) at the firewall. It shouldn't be much, but it might not need a lot. Repairs - not real easy. If it's a floppy frame, why? Is it boxed? Are the cross members welds OK? If these are OK, then you need to reinforce the firewall, do your glass repairs and hope that it doesn't happen again.

Then - of course, that firewall might just be the "one" that the builders new guy screwed up and it's one of a kind, and no one else that owns that manufacturers product ever has had or ever will have that problem

Let us know what you find