In the first photo you can see the heads of some of the plugs cut off ,but not completely ground smooth. The 2nd shows 2 of the heads lying next to the oil gallery and 1 with the head still on. once they are ground smooth ,you can drill between that row and do it again making the row solid material, then grind smooth . 3rd photo shows the plugs ground smooth. mEl did a good job of explaining the process .
I do a lot off cast iron welding ,and that is not the place for welding unless you have an oven to heat the block to at least 1000 deg f before and during the welding . Post heat to 1500 and slow cool , 1/2 hour to get down to 1200. look at the link in the first post and see some of the cast welding jobs. The reason I said use lock n stitch plugs is because the threads hooked and tend to pull the crack together rather than spread it like an iron tite plug .
Look up the website for lock n stitch and see the shape of the threads. That Ford block took about 150 plugs ,but where do you find another 427 side oi;er.
Keenserts use standard taps and they are much thicker wall than a helicoil thus spreading the load over a larger area.