Randy Shivak makes new M79 receivers and therefore do not need to be classified as a DD on a form one. He also makes both 40mm barrels and 37mm barrels for his receivers. If you get the 40mm barrel you will need to DD it on a Form1. If you get the 37mm barrel you don't, however you will be restricted to flares, smoke and gas only. If you want to fire any round which are classified as AP like stingers, flaschettes, baton rounds, etc you will still need to DD it. You will also need to get a complete original M79 parts kit to finish your project.
I believe but don't remember for sure his receivers cost $700 and his barrels are also $700. I have seen parts kits listed on Gun Broker for $600 in the past. $1,300 or $1,400 is certainly cheaper then $5,500 or $6,500 for a real M79.
MadDog