I noticed you have said "plates" a couple of times in this post. Does the 8 bolt water pump have more than one divider plate? there is some good info on clearancing the water pump in the jim hand book. I used it to do my divider plate clearancing this winter. I took a few pics of it (among other things) on this page if that helps you envision the process. there was quite a large gap before I started.
Clearancing Pics
Don't give up on your pontiac engine. Cooling problems are relatively common but they can usually be solved . just keep eliminating the weakest links and eventually you will find the problem. I agree with the comments on the fan you have installed. it does not look like it fits that shroud well. And the flex fans often have mixed reviews and success. If originality is part of your goal then get an original style correctly size clutch fan and shroud. if it is not, then get a properly sized and shrouded electric fan. there is a reason no modern vehicle comes with a mechanical fan anymore. they are just not as efficient. there are lots of good articles on the net of fitting a junkyard lincoln ford e-fan assemblies so it does not have to cost a lot either.
good luck.