Bypass hose for LA block with Magnum intake

here this explains it
"Thermostat Bypass System.— Reworking a small block's bypass system can pay off with improved cooling. Under normal circumstances, when an engine is cold, a bypass in the water pump housing allows coolant to circulate enough to avoid a pressure build up when the thermostat is closed, and to allow the thermostat to open properly. Some engine builders have found that improved cooling efficiency results when this bypass hole is plugged (with either silicone or a standard plug) and four holes are drilled in the thermostat. Typically, four 1/8" diameter holes are adequate. However, Myron Cottrell of TPI Specialties recommends four .200" holes in a 160 degree thermostat for summer operation and four .080" holes in a 180 degree thermostat for winter operation. This may seem like splitting hairs, but an engine that's overcooled produces less horsepower.The thermostat modification also improves block filling since air in the waterjackets has an escape path.

Even if everything in the cooling system is operating properly, drilling holes in the thermostat and plugging the internal bypass usually results in a measurable drop in cooling temperature. And it may cure an overheating problem if the "wrong" parts have been installed. Some race waterpumps and aftermarket cylinder heads ( a passage in the bypass circuit leads from the block to the heads) do not have provisions for the internal bypass.If the circuit is already blocked, the thermostat modification will cool things down."