Man, that is a super nice long front shroud section. Most like the ones I've built were cut in front of the first rail hole. Whatever you do keep as much of that length as you can and just square it. Having the rail and recuperator holes will be a big plus. I've used the Global Machining shells BTW, so I'm not familiar with the BRP shells.
The measurement from the rear of the cam section opening ("cam window") to the rear (buttstock end) of receiver is 382mm (15.039'') according to the MG42 armorers manual. Spec. say it can be short by .2mm .
14.562'' from rear camming piece face to end of receiver. First rail rivet hole 3.000'' from end of receiver.
Total length from end to end should be close to 37 11/16"
My overall length turned out 2mm long but had no effect on the gun.
Use your top cover for trimming the latch area leaving it proud and taking only off enough to get it to latch. Something I did to double check my grip hanger location was I used my Laffete tripod (if you have one) for a jig...pop the front shroud's mount in place and the rear mount will show where the grip hanger/mount will go on the receiver.
Hope this help you out.