Some of these versions of the St.60 had a hole in a boss that is in the middle of the exhaust. Tom Lay used to make an adapter for that engine that attached to the exhaust by that screw in the middle and fit snug around the perimeter. it had muffler ears located on the adapter that matched typical ST.60, .51 and .46 dimensions. I had Jim Lee make a few spares for me. You might check with Jim to see if he still has his noes and drawings for that. When I use mine, I use the screw as usual but apply some JB-Weld around the edges to stabilize and re-enforce the joint.
Type at you later,
HAPPY NEW YEAR!!
Dan McEntee