For that system a relief valve is a must. You can pipe it back into the radiator overflow reservoir that way when not if it does pop off then the glycol/antifreeze is not wasted.
The glycol to water exchanger piped to the water to water exchanger is the safest route but on the water to water side I would reccomend a storage tank and pump for the water to water exchanger to maintain and with a few check valve and/or dedicated shutoff valves a solar system,propane system or any other heat source could be added easily for when the bus is sitting idle.
I would add a pump to the bus radiator system because your engines pump was only designed to move so much water? If you are changing a 1-row coil to a 4- row coil that could me you are starving your engine of 3-gal. A minute of cooling .
Just more food for thought?