[ERA Chas]

First, find out why it overheated then how it overheated with 2 holes in the side!
Scrap the block and you'll be money ahead. That broken "tab" as you call it, may be more trouble than you think if you have a clutch slave that has to mount there. Yes, iron can be welded, but I wouldn't want that kind of repair with clutch pressure working against it constantly. Also, the crank may be hurt from when the windows were added to the block-so don't count on it.
Get an iron Genesis SO for about $3500 (or less) done and sleep well at night. Or, if you don't have to have an SO, Keith Craft offers very nice 390 blocks in bigger displacements. BTW 4.23 SO bore x 3.98 CJ crank = 448 cid.
Good luck.