To weld cast you use Nickel rods*
Clean it up with grinder and brake clean, heat it, weld it, then immediately peen it (tap it rapidly and lightly with a hammer OR use a needle gun)
If it breaks, your retainers could come lose. IF they do then the lifters could rotate and cause problems
If you're too worried you could shell out the $$$ for a regular set of roller lifters with the bar straps.