I ran a 6' blower with 27hp at the pto and it was fine (4.5hp per foot). 64" and 21.5hp pto = 4hp per foot.
After the first cut you can always take smaller bites, and with an hst the speed can generally be controlled better (slower). As already mentioned, snow type plays a significant role in snowblowing capacity.
I personally would try to stick with the width of your tractor, or slightly wider.