Here are a few related pics from when I built my semi MG3 clone.
First shows the MG3 recuperator compared to the 8mm type (8mm type in pic has its plunger removed), but the longer tail and added spring are the main difference. That and there is sort of a hump on the rear mounting area, along with a cut out of the flange near the ejection port area for added clearance for the fatter 7.62 case.
This pic shows the modification I made to the M53 grip hanger, cutting it's "tail" recup support off.
This pic shows the MG3 type rear section and how the lower buffer tab is cut out in order to support the longer MG3 type recuperator.
The grip support/hanger is riveted and has a bolt that goes through holding the rear of the recuperator. Here is a pic of mine after it being located prior to welding. The hole is for the recup bolt which goes through the grip support, bolt blocker and through the recuperator. I just welded the grip support in place, welding the rear, sides and plug welding the rivet holes (over top of the semi bolt blocker). Notice the square hole in the reciever is no longer needed in an MG3 build because the tail will not be used.
Hope this answers your questions.