Is it best to remove the front receiver scrap piece before you remove the barrel or after?

woodsman, your best bet is to use a bench grinder on the revit heads.  Being careful to not grind on any exposed area of the trunnion.   Also, you can use a dremmel with a cut off wheel  to grind down the revit heads.

Next, use the cut off wheel to slice a single cut on the BOTTOM of the old receiver, Starting at the Muzzel end and working back to toward the body of the receiver.  Once you have sliced the receiver in half (length wise), you can use a set of pliars to pull the receiver away from the trunnion.  

Send me you email address and I'll send you photos of this.  

I like to do it after.

Leaving it in place gives you a little extra metal to put in your vise and shields the trunion from crunch marks.  After you're done pounding etc then take it off.

Want to do it after...

Take the barrel pin and barrel out, then use the receiver shell to allow you to grind the heads off the rivets, that way you grind the old receiver shell....

then just tap the rivets out with a punch and hammer and pop the shell off.

If you're carefuly after that you can slice the shell down the middle and use one side for a hole template in your new receiver.