Most of the rivets were simple to drill out. A good drill goes through aluminum rivets like butter. But with the engine in place, a few were in tight spots, so tight that I couldn't even get my right-angle adapter on them. A chisel solved that problem. Knocking the head off an aluminum rivet with a chisel is almost as easy as drilling it out.
With the firewall out, I was able to cut it and bend it:
Then I made two triangular end pieces, using my brand new bending brake from Harbor Freight:
Then I fitted them into place, drilled for rivets, and installed with Clecos.
This was my first sheet metal fabrication, and I think it turned out great. It makes me look forward to some of the other fabrication I need to do. (Battery box, I-Squared controllers, maybe more.)