Hi guys I have an 8 inch water pump pulley and 7 3/8 inch crank pulley. Power steering pulley is 6 inches and alternator pulley is 2 3/4 inch. Will I have any cavitation problems if I change the water pump pulley to a 6 1/2 inch?

Are you trying to get more circulation?

Is the car currently overheating or running hotter than you think it should.

As far as cavitation, in cooling system you need trapped air. This should not be the case with a properly function system, air will be bled out through the radiator cap during expansion.

Just trying to get more circulation when sitting in traffic. Just unsure about pulley ratios

It is natural to think more water flow will solve over-heating. But actually, a slower flow will let the water stay in the radiator longer, releasing more heat. I had a 1967 Firebird 400 HO, with manual steering. The only fan belt was for the alternator and water pump. It was the biggest water pump pulley I ever saw. But it came with a huge 4 row brass radiator. It never got hot. The condition of the radiator, engine water passages, water pump, and air flow are the real culprits for over-heating.

Do you have the 400 specific air blocker plates at the top of the opening? They help. Do you have a good radiator shroud? Is the fan located half in/ half out? Do you have a working multi-blade clutch fan? I used to steal them off big Pontiacs for the better cooling. I also like to clean the engine block with Cascade dishwashing machine detergent. It gets the crud and rust out. As good as brass radiators are, the new aluminum ones are as good if not better. You just have to use distilled water so the aluminum and iron don't react.

Remember that a lot of small problems can add up to over-heating.

I drive my junk here in AZ when ambient temps are 100-112 and don't have any rising coolant temp problems. This was with 4 row copper brass radiator, Hayden heavy duty fan clutch, 7 blade fan, 4 1/2" water pump pulley.
Here,s the check list...
1. water pump divider plate clearance. You want the impeller as close to the divider plate as possible. Use hammer and hammer the plate all around the part that sits over the impellers and account for thickness of gasket. Also make sure you have the tubes that fit inside the timing cover and good rubber seals to seal the divider plate.
2. Check the radiator is not plugged with corrosion. Radiators do not last forever.
3. fan shroud is a must as is a thermostat.
The big factory water pump pulley doesn't spin the water pump fast enough for idling around traffic. Never use tap water it has to many minerals. If you use distilled water without any commercial anti-freeze/ coolant you must use an anti corrosion additive.

More water flow is always better for removing heat. Heat flow is governed by delta T so the more hot water you can get into the radiator the better the engine will cool. Thermodynamics.

I've had three different diameter water pump pulleys, didn't have overheating with any. They all kept the temp at 180, even in 100+ temps.

One can get cavitation in water with no air bubbles around when in low pressure areas behind blades. I don't think you have anything to worry about in your water pump.