If you don't have a barrel with a fluted chamber you have a very expensive single shot. HK will not sell you one and the only 2 makers in the world were charging around $600.
Guy I worked with at T/C tried that, but he had a custom barrel made at the shop.
He was about $1500 into it when his wife cut off his allowance. (He paid around $200 for a PSG 1 pistol grip)
If you are going to build a rifle build an AR 10. The design is more builder freindly.
"Proper tooling" for bending a receiver is a Press break and will cost $1500. You can get the job done with 2 2X4's 2 C clamps and a soft face hammer, can even use a chunk of fire wood if you don't have a hammer

There is also the issue of welding the trunnion, this has to be right, the righter it is the better, what good is a scoped rifle if the barrel has a half degree of kink in it's base ?
If you have to find out for yourself, try an AK first, it's much cheaper.