Rick,
The problem you're having should not exist, if everything in your cooling system is working properly. I have a similar motor to yours (427 cubic inch, Fontana aluminum block, SVO aluminum heads, etc.). A week ago, I was driving around in Placerville/Shingle Springs, CA in 100+ degree temps and my coolant temps were exactly the same as they always are. I even got caught in road construction, where I was stopped and idling for about 20 minutes. My temps didn't rise by a single degree from my normal operating temperature. I have 2 temp senders and gauges, so I know the readings are accurate. My max temp is 186 on my Holley EFI gauge and 190 on my OEM gauge. I also use an 8 psi rad cap without an issue.
First things first: are you sure you have the correct 351 Cleveland thermostat and bypass plate just below it? You have a Ron Davis radiator with dual 13" fans mounted in a shroud. Do both fans turn on at the same time or does one turn on before the other? If it's the latter, do you have a baffle between the fans that isolates the fans from one another? If you don't, you need one or you need to make it so both fans turn on at the same time. If there's no baffle and only one fan is "on", it simply sucks air through the opposite fan opening, not through the rad. PWM control of both fans is the best way to go. Pantera Electronics' fan controller works this way. PWM varies the fan's speed according to coolant temp.
If you're still looking to improve your cooling, George's self bleeding mods work great. Just because you get all of the air out of the system doesn't mean more air bubbles can't form through tiny system leaks, hot spots or pump cavitation. These mods are constantly bleeding air out of the system. I even have bleed fittings in the rear of my cylinder heads.
There are other things you can do to improve the efficiency of your cooling system but they require fabrication work and re-plumbing of the system. When it comes to cooling, fluid velocity is everything. Your mechanical water pump was never designed to pull coolant through 20 feet of plumbing! I have an electric booster pump at the outlet of the radiator. It makes the mechanical water pump's job easier, helps prevent cavitation and the air bubbles that result. It can also circulate coolant after you shut the engine off, to help eliminate hot spots. It really helps at idle too, as it flows more coolant than the mechanical water pump does (at idle).
Aftermarket rads, designed for Pantera's, seem to always be dual pass rads. I assume that's because it makes it easier to connect to the OEM plumbing. A single pass rad is a bit more efficient as it reduces restriction to flow. The more time that coolant spends in your rad, the less efficiently the rad works. Tip: Ron Davis will sell you a single pass version of their dual pass rad. You can even fit two 14" fans but you'd have to have a custom shroud made.
Another interesting mod is John Taphorn's remote, by-pass thermostat mod. This mod removes the thermostat from the motor and puts it at the inlet to the radiator. A bypass line is added to the system so coolant by-passes the rad until the thermostat opens. The benefit of this is increased velocity of coolant through the system, particularly during warm-up. John used a BMW remote thermostat but IPSCO now has remote by-pass thermostat housings. The GT40 crowd seem to have embraced this mod.
Finally, aluminum block based engines can benefit from an oil/water heat exchanger. Aluminum blocks grow quite a bit from cool to hot. They are usually built with tight clearances cold, so clearances are normal when the block is at normal operating temp. An oil/water heat exchanger cools the oil when it's hot but also serves to heat the oil when it's cool, as coolant warms up much more quickly than oil does, particularly with a 10 quart oil pan. This helps to get the block up to normal operating temperature more quickly.