Woody; I think what will determine the GPM is the size of the coolant pipes to the front of the car. I know you know the diameter of your pipes, but my thought was for 1.5" id pipe size you could flow 35 GPM at 0 psi. Attached is a chart that can give you various pipe diameters.
https://www.hy-techroofdrains.com/water-flow-through-a-pipe/