Another possibility is that you have clogged or restricted water passages inside the block. Is this a new build or have you recently put this pump on an already in service engine?

If the later is the case, you might try a thorough flush of the engine by a good radiator shop followed by a new thermostat.

The high volume pumps, like yours, really put a strain on things until that thermostat opens up. You may want to drill a small (3/16") hole in the outer edge of the thermostat to allow water to pass on start up and relieve some of the pressure.

I'm curious, was your engine overheating before the new pump? What was the reason to go to the high volume (and higher pressure) pump?

I'm sure several others will chime in with some ideas/suggestions, so all the info you can provide will help in finding a quick resolution.

Regards,
Glenn