At the risk of sounding like an azzhole, you sound like you are new to this. IF you are interested in the legalities of building a gun, may I suggest you start out small, like sporterizing and such. A MG34 is a HUGE undertaking and legalities should be the least of your concerns. But here in a nutshell is your answer...
FEDERAL.....you can build a weapon providing its not full auto, has a bbl over 16 inches and fires from the closed bolt position. Make sure you know 922 Compliance laws as well. There are several other nuances, but thats the quick version.
State.....FInd a local builder on this site and PM them, go offline and discuss it. IM from NY, our laws suck, but Calirornia laws are just retarded. Just remember this. Its really easy to cut corners and bend the law. Nobody ever gets caught for building non compliant guns...but do you want to be the first guy to get butt hurt in jail because you were running a 50 round belt or a pistol grip? Know your local laws. If in doubt, walk away.
My soultion for everything "Get a bigger hammer or larger caliber!"