Thank you both for your replies!
Mike - I follow until the 'take each of the outputs to a group (20 Max) of shift registers' part - does this mean to replace the clock line (that's currently being passed/chained) with one of the outputs from the inverting buffer every 20 boards or so? What do I do with the 'outbound' clock line between the 20th/21st boards?
Or do you mean take the clock line strait from the Arduino and put it into the buffer? And then distribute the outputs to groups of 20?
Sorry for the confusion.