It's really very simple. The Chinese barrel is out of spec. If you change both the barrel and the bolt, you won't have any problem fitting the parts onto a Chinese reciever. I have a Polytech, and rebuilt the whole thing with USGI parts and had not a problem with any of it fitting together.
I tried various combinations of Chinese and USGI parts while I was doing the build (just to find out myself) and there wasn't much between the two (USGI and Chicom) that would fit together. Some did, but not anything really important.
Mount a Chincom barrel into a Springfield M-1A reciever and the USGI bolt isn't going to fit there either. The problem's not with receiver geometry. The problem is trying to get two parts that have VERY little chance of working together to fit.
The answer seems to elude many, but it's the same as asking why a 1/2 inch socket doesn't quite fit a 13mm bolt head. It doens't fit because it's the worng size. Simple as that.
People want a cheap way out, so companies provide that service to make a buck. They'll modify your USGI dimensioned receiver to accept a USGI bolt headspaced to an out of spec Chicom barrel. It costs less, so people do it.
If you use a USGI dimensioned receiver (Chinese forged) and a USGI bolt, and a USGI barrel the most amazing thing happens. It all fits! Unbelievable!!
The best thing to do is make sure the Chicom gun is really out of headspace. Many people don't check it correctly for a military rifle and what they think is out of headspace is actually fine. If it's good, shoot the crap out of it. If it's not good , the best thing to do is have a new USGI barrel and bolt fitted.
Having your receiver modified to take the worng combination of parts is not the route to take IMO. But, it's just my opinion and I don't mind what anyone does to their own gun.
Above all think of total cost. If it's going to cost you as much to fix a broken Chicom gun as it is to just buy a US made gun, the choice is obvious. I wanted a forged receiver, and I wanted to do the build myself, so I went the route I did. I also own a match modified M-1A, so the Chicom is the "blaster".
My experience in the Polytech build almost mirrored exactly the info on the Fulton website just FYI.
Ross