Re: OMC 800 intermediate hose.......
Routing of the inclosed cooling system hoses are probably brand specific. I have a San Juan Engineering system on my 302. That system included a thermostat housing that replaced the OEM thermostat housing tree. It is very similar to an automotive style houlsing. You no longer need to route the impeller flow thru the engine.
My hose, from the IH, goes directly to the raw water inlet at the bottom of the heat exchanger. The raw water exits the heat exchanger and then "T"s out to the front end of each log style OEM exhaust manifolds. From there, of course, it goes thru the manifolds and exits out thru the IH.
The engine coolant then circulates thru the inner core of the heat exchanger and right back into the bottom of the engines water pump.
Hope that helps. Don't know anything about a heater core. Maybe you need to isolate it for now?