for a 416 with no AC.

whats the up/downsides of each.

I thought the 6 blade was for AC but I cant remember (I thought it was just the opposite of what you'd expect)

It is infact opposite of what you would expect.
8 blade is for non-a/c
6 blade is for a/c, but remember the pulley is smaller so it over drives the pump for an a/c car.

There really isn't a draw back, except if you use the non a/c pulley for an a/c pump, you may not get enough coolant flow.

get an 8 blader & consider tacking on a flat disc to the impeller.

the engine cools very well right now. Not sure if I need any extra help in that department

Like said above the 6 blade is for AC and 8 blade for non AC. The AC cars have a smaller water pump pulley to spin the fan faster. The 6 blade is used in this case to prevent capitation. If you car cools well now but you need a new pump I'd put back on whats on there.