Since you mentioned this is on a test stand it tends to reason it will be temporary. I would just use water or maybe mix a couple of quarts of coolant together unless it will be setting out in the freezing weather at some point. There is enough clearance inside the waterpump to allow it to be filled from the top of the manifold from one of the heater hose connections. Use a funnel and slowly fill the system, it will fill the block, part of the radiator, heads and manifold, it will also purge the air at the same time, leave the cap off of the radiator while doing this, when the water/coolant reaches the top of the hole using to fill it replace the threaded plug and continue filling from the radiator. Once it operates and gets up to temprature after filling it this way any remaining air will be expelled when the thermostat opens. IMO it's not necessary to get overly concerned about coolant/
antifreeze mixture until it is in the car permanently. Once it is installed in the car be sure the coolant you choose is compatible with the rubber and elastomers used in the headgaskets you are using. Some of the coolants are designed to be used with certain types of gaskets/rubber etc.
Rick