IMO repair of through cracks in the proximal 1/3 of a pattern welded barrel is an entirely different issue than the distal 1/3, where pressure is < 1000 psi, and depending on the load may be < 500 psi.
IF the barrels are welded in the proximal 1/3 I would strongly urge formal proof testing. Some options:
H.P. White Laboratory, Inc. in Maryland and Cortland, New York. They will test at 1 and 1/2 times the pressure of your intended load if asked to do so.
www.hpwhite.com
Entropy Engineering Corporation
http://www.entropyec.com/
Chesapeake Testing
http://chesapeaketesting.com/service...blast-testing/
Dayton T. Brown
http://www.dtbtest.com/Default.aspx
Please let us know if you proceed and how it works out.