Well i too am no EXPERT by any means nore am I a Profesional BODYMAN that works every day in a Paint and Body shop or Colision repair shop, HOWEVER I am a Profesional mechanic with a few years of Restoration experiance.
I can tell you from experiance that your Rust is far deeper than what you are seeing on the surface and IF you remove the head liner and look BETWEEN the interior panel and the exterior panel looking at that outside skin from inside the Truck that rust is more extensive on the inside than you think! so your gonna need to replace more metal than what you think!
Ya its rusting on the Outside AND the inside! (Check it out)
At any rate making Patches for that is gonna Suck!
IF you have to ask how to metal shape Patches for that trust me its beyound your current Tallents!
IF this is your first project lets get it done the easiest way possible! BUY a whole Roof from a Junk yard Truck.
There are just too many expensive tools to buy!
Just to get it done by buying the panels, Buying everything to do everything is cost prohibitive.
That is unless you already have every thing to do Paint and body work ?????
That would include a Big Aircompressor, air hose, plumbing, air drier, water trap, regulators, Mig welder/Tig welder/Gas welder, a Good set of Hammers and Dollys, Paint guns, Paint booth, Metal saw, Die grinders, Sanders, sand paper, supplies and on and on.
Then Metal shapping
Big Stump, Hammers, Mallets, English wheel, Bead roller, Welders, Saws, Shears, screws, Clamps, Clecoes, and the List goes on and on.
Seriously while that cab corner may look easy to do Your skill leve and what you want it to look like in the end come to mind.
I personally am looking for as Perfect as I can get to original for most of the work I am doing, You on the the other hand may only be consernd with just keeping the water out. So you need to be a little more creative in what your asking I think?
Are you looking for perfect? do you have any experiance at all? what Kind of equipment do you have? What kind of repair are you looking for? What kind of money are you looking to put into this project? What kind of money are you willing to invest in yourself? for the next project?
If are you only interested in stopping Rain and snow and you plan to scrap the Truck in a Year or two? Get yourself a Can of Fiberglass Filler, some Sand papper a 2 hp aircompressor a cheap $20 harborfreight gun some cheap Paint and go at. Lots of Videos on You tube explain every thing you need to know to do this sort of thing.