[dsraven]

I highly suggest cross bracing and some dimensional cross checks to make sure you restart with something close to "square", just to make sure stuff fits together at the end of it all.
some guys have used a stell wheel barrow tub for a firewall bump, if you wanna do a sculpted bump rather than a more square bump behind the engine. if going totally flat on the firewall I suggest 16ga for added strength with some ribs on the back side at kep spots, like where pedals hand (if doing hanging pedals and a firewall brake booster). a local fab shop could likely bend you up something to match your angles. shoot some weld through primer on the edges where the metal contact ponts will be so it won't be bare metal on bare metal where rust can form. then tack it up like you want, or simply use some cleco's to hold it in place until your welded can come and weld it all in. cleco's are the best little holding tool for guys who work alone. magnets can also be a big help to hold stuff while you jiggle parts around to fit correctly.