IMO its easier to buy a kit that has the front shroud section cut off as long possible in the rear like Centerfire has (usually with the front rail and recuperator holes), this way you can square it up in the rear past the camming piece area (saves from camming piece fitting), then use an 80% receiver like a GM or BRP and square to match. which is easier to make straight and to correct length rather than piecing a torch cut receiver back together and keep it straight. Either way can be done as many threads here explain, but this to me this way is easier with the garage tools I have (Dremel, wire welder, hand files, etc)..
If you want to take another big short cut you can then splice on a squared up RTG rear MG3 stub which saves you the buffer tab hassle. Many of the torch cut full receivers are cut through the buffer tabs anyway. Keep in mind you will need a MG3 buffer and buttstock which looks much better IMO and if you go this route and you may as well install a MG3 recuperator being the buffer tabs are cut for it. If your length is correct (measure from the rear of the camming window to the rear of the receiver) as per German manual your recuperator will be correct (fore to aft) as well which is another plus.
I've only built using 80%'s BTW. I've never used a jig to try to align...if you make sure you are square and straight and put a piece of threaded rod through it to hold it together and 'stitch weld' it should stay straight if you are careful. But if its not straight before welding its not going to be after. If you do it this way you are re-welding in essence a '3 cut saw cut' rather than an angled torch cut which again is much easier IMO. '3 cut' meaning the front bushing splice/rear shroud to receiver splice and or rear MG3 stub splice. All of which should be squared. I work front to back BTW.
Just like too IMO its easy to take a M53 shroud that has the barrel bushing cut off, square the front of the shroud to length, then weld an RTG/MG3 front bushing back on to correct length; instead of cutting the whole front off and replacing it like some of the angled torch cut kits require.
Also get the RTG 'new' MG3 rails that are not drilled, then once aligned drill them and the receiver rather than trying to match up existing holes in a torch cut receiver. RTG also has the used MG3 ratchet plates...I recently bought 2 and both are nice. This saves you from trying to patch a cut up one back together and they have the handle slot already cut in them... something many newly made ones lack.
Biggest thing is keeping it all straight and as close to spec length as possible. You can sometimes be off 'a little' in one area without issues... problems later arise are when you are off in several places because of stacked tolerances.
This is what has worked well for me, others may differ....