The screws I used on the rails can be found at home depot. They are marked on the packet; Cap screw/Button socket #6-32 x 1/2" 3.5. You can also buy the drill and tap for them as a set there too; Dewalt 6-32 nc drill bit #36 SAE.
The screws come in packs of 2, you will need 10, I'd suggest 12-14 for spares. As you will probably cut a couple too short like I have. These will need to be cut down in length. I just stuck each one in a vise and zipped them off with dremel cut off wheel.
Having an intact camming section on your shroud with intact windows will be a big help for a first time builder. Thats great news and will be so much easier. The GM shells come with a camming section so they are great for builds that need them, but in your case I would just cut the front GM section off that you won't be needing. Just take enough off the rear edge to square it and no more, you want to leave all the original shroud as you can.
I like to get the slag from the torch cleaned off first and temporary install the camming peice. You can use this as a straight locating point to square the rear edge of the shroud too. I take something like a flat file and lay it on the rear edge of the CP, find the lowest point of the torch cut and square it to that if you know what I mean.
The GM set of shells do not come with a bolt blocker. I use the one BRP sells, some guys will use a bolt or something, but the BRP blocker was the one approved by the ATF so for like $20, it is worth it IMO and its a nice design.
I'm waiting for BRP to restock their bolt carriers and will need a grip stick as well. I like theirs because it allows use of an unmodified bolt with their one piece firing pin design. I have a Wiselite set up as well and it requires the bolt head to be modified (drilled and a bushing installed) and uses a 2 piece firing pin design. They both work. Of the 2 types I kind of prefer the BRP over the WLA so thats what I'm using again on my current build. Both the grip stick's hammer (AR type) and bolt carrier will probably need to be fitted (by design). It's pretty easy and we can more into that when the time comes if needed.
The manuals will be a big help for you to make sense of all this (I know the feeling BTW, been there).
On the camming piece, I'd recommend getting both the long rivet and the long bolt and nut BRP sells for this purpose (its like $5). With the bolt and nut you can install the CP temporary and test fire it and such, then take it back off to finish the gun. You can then rivet it or stay with the bolt and nut, depending on if you care about the looks. I kept my MG3 clone with the bolt and nut because my weld is under the CP. This way I can take the CP off and check the welds once in a while, at least till I get a few more thousand rounds through it.
For welding the shells I didn't use a jig. If it were a cut up receiver a jig would be needed, but for our purpose IMO a jig is not needed. The GM shells I've used so far have been very close in width to the original specs. So all I do is figure out where I don't
need to weld (ejection port, grip stick port, most of the top center) so as not to put any more heat into it than is needed.
I then bevel the areas that will be welded for better penetration, but keep the non welded seams un-beveled. This way those un-beveled areas can give it support while you take a big C-clamp to clamp the shells together. Then just stitch weld it a little at a time. I like to start near the center do about an inch, flip it over do about an inch and keep alternating pushing the welds outward which keeps the warpage and stress to a minimum.
Allow it to cool enough to hold it by hand after about 1/3 way, then start again and go back and start filling in the stitched areas skipped before. Doing it this way I haven't had any warp so far. Keep some copper on hand like a piece of flatten copper pipe in case you blow through, you can put the copper under the hole and build it back up, use a piece if tin or something to hold the copper in place under the hole. Another little trick if using a mig welder to cut down on spatter clean up I like to take some aluminum foil or scrap some sheet metal an lay to the sides of the welds. Cuts down on clean time/grinding.
I'm by no means any welder, just a garage builder with a cheap wire welder that figured out what works for me
. You can use the same process for joining the splices to the shroud. The biggest thing there when dealing with the splices is grinding or using a belt sander and getting the joined sections as square as possible for the tightest fit. This will lesson your warping and keep it as straight as possible if it fits tight and straight to begin with.
Fitting and measuring is where you want to take your time. With no mill I cut my sections a tad long with a dremel and then bring them into specs (proper length) on the belt sander a little at a time squaring them. My belt sander is a just a hand held type I flip over and chuck into my vise. I use a peice of window pain glass for a flat block to gage my fitting along with a small carpenters square on the few areas flat enough to use one on these guns.
For those spliced areas, instead of a jig I take a piece of threaded rod and some scrap metal with some nuts and washers and pull it together before welding....same with the front bushing.
BTW, I'm in the middle of a build making an MG1 clone (did a MG3 clone last year) and also have a WLA M53. I'm taking pics of each step along the way for a hand tool garage build using all the short cuts to make it easier for a first time builder. If there is enough interest here I can post it as a tutorial which should help you with yours. There are several tutorials out on the internet, most are very good. but most were done a few years ago and there are more options now and many assume you have a mill so that's why I was thinking of doing one.