Cut out new firewall. Working with SST is slow and hard on tools. Angle grinder works well for the initial cut but to smooth the contours and clean the edges requires some patience with the files. Drilling holes in the SST is hard on drill bits. The procedure is to go slow with a lot of pressure. Mounted the firewall and then match drilled the bolt holes for the engine mount. There are two AN5 bolts for each of the four corners of the engine mount, so eight 5/16" holes. I ruined only one drill bit. I consider that a success. ;-) Since I won't be using the original engine, I won't drill or cut any more holes in the firewall until the engine is mounted and the locations for passthroughs are obvious.