Some of those rifles are easily brought back to sniper form and some aren't.
I've seen some of them where the fillers screws were ground off so deep the whole area looked to be dished deeply.
Others have been done with some care "but" it depends on how they were welded over and what material was used as rod or filler.
I've seen a couple that were definitely welded electrically but most seem to have been done with oxy/acetylene and filler wire, which is very close in hardness to the receiver. The latter are the reasonably easy rifles to restore.
As mentioned, there are different brackets available. Before attempting to restore your rifle, get all of the parts together first. There used to be a KIT with all of the drills/taps/pins/bracket from Numrich and a couple of other places in the US, such as Liberty Tree. The scope and bases may need to come in from someone such as ###### Armories or one of the other banner advertisers