If you can easily get to any external crack then I would stitch it. This causes no stress to the metel, I did my jeep which had a crack in the usual place above the distributor, caused when tightening up the head,stitching is very easy, just add rad seal to cover any very minor seepage that may occur.Pouring any sort of gunk into the radiator may cause more problems. Always remember gunk may fill the existing fault, but the fault may extend itself in time.Stitching binds both sides of the fault . Hope this helps. John