[velocette]

If your intent is to in some way return this firearm to shootable status, there is one answer. DON'T do it.
As MP Driver noted, you have no way of knowing how hot the RECEIVER got during the barrel welding process. Therefore, you have no way of knowing what has happened to the heat treating of the receiver, or what parts of the receiver have had their heat treat changed.
A Kaboom of a high powered rifle is not pleasant and can be extremely painful or fatal.
Leave the rifle as a wall hanger. Go to Gunbroker or the equivalent to find good non modified rifle to work with.

Roger