The vacuum advance will help the car run cooler, get better gas mileage and feel more responsive/peppier at low to mid throttle angles and light load cruising Where the motor can tolerate 50 degrees or more advance. It does not come into play at idle in the stock configuration with the thermal vacuum switch until temp goes past 230. then the switch changes the vacuum source to manifold vacuum which at idle advances it to whatever the full setpoint of the canister is on top of the static timing setpoint to cool the engine down. Once it does this the switch reverts back to ported vacuum. I never understood the thinking behind buying an expensive distributor that eliminates the vacuum advance function in a street cruiser but hey to each there own. Back to your overheating at idle problem you never said what size crank and waterpump pulleys are on your engine, it's important. Here is a factory bulletin from PMD back in the day talking about overheating and those masticated seals I was talking about earlier https://firstgenfirebird.org/f-a-q/category/cooling-system/ As far as the fan depth I think it looks fine and sticking it in deeper can actually cause the air to get trapped longer in the shroud just circulating. I don't have any experience with champion radiators and I know they claim the 2 core is rated up to 400 HP but I would be suspect of that and if putting a car together using there product would have at the very least gone 3 core and knowing me most likely overkilled it with 4 which they claim is good to 800 HP. Some more things to check, is the mixture 50/50? it makes a difference. change the thermostat just because it's cheap and easy and will eliminate any questions about it opening fully. Make sure and drill a 16th of an inch hole in the flat part to eliminate the dreaded first start air pocket. On the fan clutch shut off the engine when up to full operating temp and try to turn, it should be very hard to turn. Make sure the radiator cap is holding it's full 15 pounds if you don't have the tester an auto parts store will do it for you. If you are at all suspect of the fan clutch find a fellow Pontiac guy and borrow a factory flex fan to try out it's quick and easy. Make sure the fins on the A/C condenser in front of your radiator are not mashed and layed over on both sides by someone who was heavy handed during installation, The same goes for the radiator. You can buy a fin rake to remedy this. On your timing, check to make sure the outer ring of the crank dampener has not slipped on the rubber ring causing the timing to be out even though it shows correctly with the timing light. To do this just stick a wooden dowel or something like it into the #1 spark plug hole and turn crank until you physically verify it's at tdc and check that the line on the dampener is lined up with 0. Many a mechanic has been fooled by this when not excessively out. Lastly I assume the engine timing curve and setpoints were done on an engine dyno when it was built? was the fuel mixture optimized at the same time?. If the idle circuit is to lean it will contribute to higher temps. The best thing you can do for your car is to find a good engine builder/tuner with a chassis dyno to set everything up optimally , it makes all the difference in the world. Do you know what brand and color paint was used to paint the engine?. At least on my computer screen it looks much closer to the factory color on the original engines I have torn down compared to the more blueish colors that a lot of company's sell today.