More ignition timing actually COOLS a motor, especially at idle. Low ignition timing will cause it to heat up since the delayed spark causes incomplete evacuation of exhaust gases during the exhaust stroke, leading to accumulation of heat. I've seen headers start glowing orange at idle due to low timing.
First thing is first, as stated, you want vacuum advance for a street motor. Race motors don't use vacuum advance because they usually operate at WOT (wide open throttle) at which point vacuum advance is irrelevant, anyhow.
Think about a bone stock 1978 or 1979 Trans Am w/ a 400 W72. These motors made low compression, and also used EGR which actually cools combustion temps due to the introduction of inert gas into the combustion mixture - these engines had a lot of timing to make them livable and so they would not overheat with all the emissions crap. Base ignition timing was 18 degrees, on top of that you have vacuum advance cans that added up to 25 degrees of advance all in by 12" of vacuum. The advance cans were originally hooked up to vacuum modulator valves calibrated to provide 9" of manifold vacuum at idle, adding on average 17 degrees of additional timing at idle, making a total timing at just IDLE around 37 degrees. That's around what my '79 400 is at, and I'll idle at thermostat temp all freakin' day. This is also why the mechanical advance setups on these cars were for slow and gradual with heavy springs - you would only get an additional 18 degrees or so by near redline RPMs. If your car has a big camshaft in it, it's going to like more idle timing, anyway so maybe up your initial timing a bit more. Unless you're making mondo compression (unlikely for a '75 400, even when "built up"), I doubt you're going to detonate. 14 degrees initial is about average for any engine, much less a Pontiac, and the fact that you're getting hot during inclines would lead me to believe that it's detonation, but with 14 degrees initial, I doubt that, and when going up inclines, the engine is under a pretty decent load which will rule out vacuum advance.
But you say you have vacuum advance on, so maybe that is not your issue. Running lean is sometimes a reach, I doubt that's why you're running hot. It's usually a lot more simple and obvious. How's your radiator? You said it's new, but is it sufficient? My 1979 with a 400 had a cool aluminum Champion radiator in it when I got it, but whoever bought it did not do their research. It was a 3 row radiator, when the original factory was a 4-row copper/brass, which would have cooled much better and had more coolant capacity, so they technically DOWNGRADED. I switched to a thick oversized 2-row Cold Case and haven't looked back. You said you took care of the water pump, but did you make sure to keep the clearance between the divider plate and impeller as tight as possible? Are your belts slipping? You can't always hear them slip, and a slipping belt essentially causes the water pump to just freewheel and not pump water as efficiently. You said you replaced your fan clutch, so you should be alright there. Next time you start the car up cold, remove your radiator cap before you start the car (have a rag ready, because the initial spooling of the water pump will cause the coolant to splash out). Keep the cap off while the car warms up and the thermostat opens and look for excess bubbles in the radiator. Maybe there is just air in the system and it needs burping. These 400's take a LOOOOOOONG time and repeated heat-up and cool downs to eliminate all the air in the system. I'll have mine idling for up to 45 minutes, even then, it takes several drives for the coolant level in the expansion tank to stop lowering and achieve equilibrium. If the bubbles never do seem to stop, though, or if the engine quickly overheats in conjunction with the bubbles, it may be time to purchase a combustion leak tester and check to see if you have a blown head gasket. Losing coolant is also a sign of a head gasket failure. Also, what temp thermostat do you have? These cars came with a 195 degree thermostat from the factory, and seeing temperatures of 205-210, even as high as 215 is not unheard of in the heat of summer. People make fun of them all the time, but sometimes ignorance really is bliss with idiot lights - they don't even go off until 240, and GM documentation of the time has verified that actual damage is not likely until coolant temps reach 260 or beyond. We freak out so much seeing temps past 200+ when in reality, that's the norm. And then most simple of all... are you actually running those temperatures? Get an infrared thermometer or a mechanical gauge; verify the temps.
Heating up at highway speeds is normal since the engine is operating at a higher RPM, meaning more work and heat produced. But then, it should also eventually stop and level out not too far past the thermostat temp rating. If it keeps rising and rising and rising, then it's usually a radiator or coolant flow problem. I'm guessing you have the chin spoiler installed correctly, right? Because that piece serves a second purpose besides just looking cool - it diverts ram air into the radiator to dissipate heat from the coolant.
