By going .20 over they are leaving room for a second .20 rebuild.
I beleive that you can go up to .40 over, anymore than that you start running into having too little room between the bores and the associated webbing flexing/overheating.
I agree with Deep6. The last short block I purchased from Ford came .40 over, I beleive that is the max, but you should safely be able to go that far if you have to since the factory does.
Jeff