my reference is towards stitch / dowling. ie, see if you can find places to drill thru the broken off flange into the parent metal and then prep for a dowel or threaded dowel (stud ) machine the area of the flange so you can put a pal nut on and literally bolt he part into place using a couple studs and nuts and then weld up and weld the studs head in or grind off, etc.. etc.
Think of it like rebar in concrete. ie.. something bridging the 2 parts.
iv'e done it on cracked shotgun but stocks using dowel rod to secure 2 parts before gluing, then trimming dowels off, sanding and re-staining.
have done it on limited basis with metal repair.
as for the jig.. weld up something that can bolt to the block and mate with t he flange to hold it true once you have it doweled ( if you go that route, or otherwise hold it true and tight, and then do the welds. this will prevent the flange from drawing one way or the other and thus being deformed and not stressed and try to re-crack along the repair when you bolt it up and are forcing pressure against the bent part.
ps.. go into this repair knowing you have a bad block, and think of it as a practice or learning experience.. if the repair fails.. you still learned some techniques.. if it works, you went from a bad block, to one that may be bad again.. at some point
or may not...
one last thing. if there are areas that you could add external stiffeners on either the inside or outside, investigage that and then find some plate or thick angle you can heat and form on an anvil to match contour perfectly, and then find appropriate places to bolt it thr/to the block and damaged flange. BT/DT using a 'sandwich plate on a broken ear on a casting /flange where I had access to both sides .. ie.. to the top/outside of the damaged flange and to the back side of the flange it mated with.. used a plate on either side and long bolts thru.. plates were shaped like the flange and extened thru a couple more bolt holes down each side to spread out streeses to areas past the damage. I see repairs liek that on loader tractors all the time where soemthing breaks out a loader attach pont in a piece of cast, adn they scab a subframe around it using other fasteners to carry the weight , plus the repaired area.
soundguy