Assuming that this is to be on a Closed Cooling system, here's how I like to explain this.
The entire closed cooling system is under pressure when at operating temperature.
Not much.... 10-15 psi or so +/-!
When supplying either a cabin heater or water heater, it requires a mild pressure differential between supply/return.
On the SBC, and many other V-8's, the place for the supply is just upstream from the thermostat.
Since the thermostat is holding back coolant (from what the Circulating Pump is charging the block with), this builds a pressure differential between this point and the suction side.
IOW, this is the perfect location for a supply.
The engine circulating pump ports are suction, not supply.
Typically, the Stbd-most port on the SBC circ pump, is the port used for the return.
When we connect this way, our system will be balanced and won't require what's called a "Puppy Pump".
We shouldn't need a Puppy Pump unless the distance to/from is too great.
I like to see the hose ID at 5/8" minimum, 3/4" would be better.
I'd supply the water heater first, then loop in series to the cabin heater, and then make your return.
If you want to shut one or the other down during summer, then you can valve this, but it gets a bit involved.
It can be done, however.
Edit:
I got to thinking that you did not mention whether this engine was Raw Water cooled, or equipped with a Closed Cooling system.
While I was suggesting plumbing for a closed system, the same applies to an open system.
If open, don't forget that this now becomes part of your winterizing procedure in that the exchanger and/or heater core must now also be drained.
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