In my opinion, from building lots of "box receiver" MGs, it is mandatory, especially if it is your first, to have major internals, topcover, barrel support and grip frame on hand to constantly fit into and onto the box as you assemble it. Mocking up the box with plates, top and bottom with well fitted screws in the rivet holes to see how it falls together and assess what needs to be done to insure accurate assembly. Screws with unthreaded shanks are best as substitute rivets for this purpose. The bolt heads will have to be worked so that when assembled from the inside out, they won't interfere with the assembly of lock frame and trigger frame. Trunnion cross pins can be duplicated with bolts. Assembly of grip frame and barrel to lock frame with barrel support added and assembly into the mocked up receiver will tell you a lot. Add on the topcover with latch and install the bolt and recoil spring, and you will know how accurate the assembly is and likely to be.
FWIW