I've posted this before on another thread but Permatex High Strength Automotive Grade Sleeve Retainer(NAPA # 765-1146) works extremely well in my experience for fixing a mild steel shim to the hook.
After fitting the shim to the hook degrease both the shim & the hook, apply a drop to the hook, put the shim in the hook then assemble the bbls & forend on the action & let it cure for 24 hours. Put some grease in the action mortise & on the sides of the lumps before assembly to assure that you can easily get the gun open if any excess sleeve retainer is squeezed out from the shim/hook joint.
