There are only general installation instructions because they sell those to hotrodders who are creating stuff and don't need no stinking instructions.
First off, do you plan on moulding it all in flush? If that's the case I would weld it in just as you have it, screwing it tight of course. On that one triangle area that sticks out just make a little piece of metal to fill it and mould that baby in.
The one tip I would give is screw it tight all over, welding up the screw holes is a breeze. But the big tip would be to weld small tacks just a quarter inch or so with a hot setting. Do some tests, including destruction tests on scrap so you are confident with each of your welds. Get a piece of metal and put a fold in it just like your cowl has and lay a piece of scrap over the top just like your new firewall and weld it just like you will be welding on your car. Do a few welds and look on the back for penetration, put the piece in a vice and try to pull them apart, get comfortable so with every one of your welds you will be confident they are perfect.
I will weld something like that with a pretty hot weld, that way you make a QUICK weld and not heat up the surrounding metal much. Cut off the wire with EVERY SINGLE weld so you have a nice sharp wire to hit the metal and start and arc, that with a hot setting on the welder and you can make a super quick, penetrating weld.
Skip all over while welding in this firewall. Make a short 1/4" weld over on the right side, then on the left, then up on the middle upper. Then WALK AWAY and work on something else to let it cool. It will NOT cool if you sit there and stare at it.
WALK AWAY after a few welds and leave it the heck alone to cool on it's own. When you can come back and place your hand on the weld and it's no warmer than the metal where you haven't welded, you can make three or four more welds, again, WELL SPACED, one side to the other, then walk away and let it cool on it's own.
You don't have tickets to the theater in an hour right? Good, because you want to take ALL DAY or more welding that firewall. I am not kidding, welding that in should take ALL DAY LONG. But of course you are going to be working on something else while you let the welds cool so you will get a lot done today.
If you weld it in like this you are likely to have very, very little warpage to that every easy to warp flat firewall. If you don't do it like this and weld long welds and cool them with rags and crap like that you will have a warped mess that will require a bunch of bondo work to make look presentable.