I am having my 1968 428 rebuilt and thought (guessed) that I had an 8 bolt water pump. I went and picked it up to repaint and figured out it is a 69-70 cast propeller and pump and matching backing plate and timing cover. To my surprise someone has already swapped it to an 11 bolt! It looks to be in great condition. I have no idea how well it works since I only have about 30 min of driving time. Should I go ahead and install a flowkooler or just stick with this one and modify the backing plate to fit a little closer to the impeller?

Does it cool adequately? If so, then why change it? Since it is a '68 428 in a '67, I'm assuming you aren't overly concerned about numbers matching? If you want to replicate as closely as possible, I would say switch to an 8 bolt.

I have no idea how well the water pump works. I have only drove it a few miles and it was running a little hot. I have since installed a new radiator and electric fan and the motor is being rebuilt. The impeller looks to be in great shape. How well do these water pumps work is more what I am wondering? I care nothing about matching numbers. Do the flowkooler water pumps work so much better that i should go ahead replace while I am having the engine rebuilt?

Stock water pumps worked for many people for decades. If you fear an ooverheating issue then I would change it now since it is convenient.