Norman,
all good questions!
The plates are flame cut and pre-cutting the holes is not really an option as they have to be drilled to fit on assembly.
The rivet holes need to be close tolerance on diameter otherwise the rivets will not set correctly.
We use MIG when we are going to disguise the weld, i.e sectioning a plate in but where welds will be left visible, we generally use stick. Contrary to popular opinion, some of the larger weld runs on Shermans were done with MIG or at least a wire fed weld, in the factory. I have seen pictures of the M4A4 production line that shows this and there was bits of wire sticking out of one of the welds in my Sherman.
I have had some sucess in getting MIG to look like stick but generally we use the original methods.
The problem with using an old plate to replace damage is the welds. It is possible to distress them to look like the plate, indeed we rebuilt a Sherman turret that needed a whiole new bustle built. This was originally a casting obviously and we made it up from plate and tube sections. A lot od welding, sculpting with a grinder and then finishing with a die grinder and needle gun left it nicely textured and contoured. A lot of work though!
Frankly, unless the plate is very pitted, when blasted and with a few coats of paint, new looks much like old. You cannot see the join in my Shermans side where there is a three foot section of new plate.