I have a front suspension with rear-steer rack and pinion (Jaguar) and rather than modify the oil pan, I pushed the engine back a little. It is a 5.3 LS. With the stock firewall the valve covers would hit the edges of the stock 'doghouse'.
My first thought was to buy an aftermarket firewall, but I didn't care much for the aesthetics.
I'd heard of guys buying a steel wheelbarrow - don't laugh - and cutting it up to use as a wider doghouse. I'd seen this done and it looked good!
As far as filling holes, the guy doing the metalwork on my truck said it'd take him about the same time to fill the holes as to fab a new firewall. Here's what it looks like today (sorry to anybody that's already seen this on my build thread or other sites- seems to be a pretty common topic)
This firewall is make from flat stock on a bead roller and spot welded into place where the factory one came out. The factory 'hat' sections inside the cab were salvaged and reinstalled as well as a new reinforcement that ran horizontally from one side of the cab to the other
And from the inside
