Too bad about the fan shroud, as a replacement is around $100. You can get some plastic epoxy and glue it back together. I did that on my GT, and it has been holding for the past year.
About the fan, there are many different 1968 fans, and some have a larger diameter. My car (289 cu in with air) has a fan that flexes as the engine speeds up. The part number on this fan is "OF C8 SE-B".
If your car has factory air conditioning, it should have the 24" radiator and this radiator rides on rubber cushions. The cushoins can be worn or missing misaligning the radiator. It could also be a worn motor mount, as mentioned earlier in this forum.
Finally, the only way to get the shroud off is to 1) remove the shroud from the radiator and slid it back toward the rear of the car, and 2) remove the radiator from the car. Then take the shroud out around the fan. It's a pain!
Once again, sorry you broke the shroud.
Russ