I would v out the crack with a grinder to a bout a 1/2 '' both sides and deep as well.Then i would tack it on all four sides first the run pass's by goin oppisite sides one pass at a time so it doses'nt pull out of square and repeat this till its welded out.7018 is not the rod for cast , i think its a 152 cast rod i cant remember the brand name . You'll get out of using a pre and post heat it digs l;ike 6011 and use's the same heat.They work great on cast I would also sweat the crack out just enough to get the moisture out when your tackin about 200 degrees.