I prefer metal receivers and they are required to mount a wood stock so I am retrofitting my 1977s with metal receivers. There are basket case (beyond repair) metal receiver guns out there that can make for good parts donors. Many times these guns can be purchased for less than the receivers sold individually. I keep an eye out for those types of guns and have been able to buy several in the $20 to $30 price range. They are a great source for metal pump arms for 880s and 822s also. I found one that the barrel was bent 90 degrees but the receivers and pump arm were no worse for the ware.
   So to fit metal receivers on new style valve bodies the breach area of the receivers must be filed back. That is the only part of the receivers that need to be modified. I use a couple of files to file the receivers back until they close around the new valvebody. File a little, test fit, rinse and repeat until they fully close. I use a flat and round file to slowly remove metal from the breach area.