I have a '68 400. Never had an issue with it running hot - but I avoid running it in stop and go traffic when it's 90+.
That said, needed to replace the water pump. Based on comments here I landed on the FlowKooler 330-180. I also picked up a new high flow (180) thermostat. The mechanic messaged me this morning to advise that "... having trouble with it running hot. The flow plate made contact with the new water pump so I left it out.". I know this is not a lot of info but I thought I would post to see if anyone else had that initial challenge - running hot - when installing a new water pump / FlowKooler. Any thoughts I can pass on?

You can’t be leaving parts out of your water pump.

Yep. Lots of documentation here on the site about now not only is it essential that the flow (restrictor) plate be installed but it be as close to the impeller as possible without hitting. Like within a feeler gauge width.

You have to tweak the flow plate to get it to fit correctly. Sometimes it takes a bit of hammer and dolly work. (Sort of).

1) Find a new mechanic. That’s Pontiac cooling 101. ...divider plate to impeller clearance...

Flowkooler 330-180 is the number of the thermostat not the pump, The divider plate has to be in there for the pump to work. Best with about 10 thou. clearance from the impeller blades. The Flowkooler impellers are machined, not cast like the originals, that may have caused some interference with the plate. The plate can be shaped with a small hammer and a bucking bar to fit correctly. The water will be spinning around and going nowhere without the plate.

AYE without the plate your impellar is just spinning, it's a centrifugal pump, water comes in the center and gets flung to the outter edges, ( see pics above)
without the plate the water has no direction, hopefully this did not ruin your motor.

Kinda weird, I installed the same FlowKooler pump and my divider plate was a fair bit away from it. I had to hammer it a bit so it would be closer. Still don't think I got it close enough.

Can't find the pics of mine, stock cast pump with stock plate. I got mine fairly tight then thought it may change shape a bit when hot so I backed off to about 10 thou. Seems to pump pretty good.