That's a molded hose that goes to the heater core. When I did my roller rockers, I replaced it with a new one from Dan Newman.
I'm not sure if regular heater hose will make the right bends without it crimping or going up against the valve cover or cylinder head.
It's easier reaching the heater core end of that hose if you remove the windshield cowling.
1998 4.6 LX
1990 5-speed SC