You can't drill an outlet into the manifold at the front and back water ports to the heads?, most stacks injection donot have crossovers either, mine included. We put lines from the back to the front and tee them into the upper hose on the rad. and run the electric pump from the bottom hose. I've ran several 377" motors both on gas and alcohol with no overheating problems. In fact the engine in my Malibu is a 377" but it's on alcohol, which doesn't need any cooling between rounds. However when i did run gas with a 377" with a tunnel ram, i turned on the pump and fan on the run and made the run 1/4 mi. and drove it back too the pits, i let the fan and pump run a few min. and it was ready to run again. Yes i've alwayed filled the block to the bottom of the water pump holes and still do today.
JMO
Zip.