You have aluminum heads that DON'T have accessory bolt holes? I didn't know they made such a thing.
What brand of heads? What part number?
No accessory bolt holes = you're screwed.
In-focus, nicely-cropped-and-sized photos of the heads, and the brackets you're trying to use?
So drop a thermometer into the radiator after removing the rad cap, and find out what the temperature is. I'd expect 170-ish at the rad cap, since you'd be on the cool side of the radiator.
https://www.amazon.com/Robinair-10597-Dial-Thermometer-220°F/dp/B000O3M5CE/ref=sr_1_3?keywords=Robinair+thermometer&qid=1582511743&sr=8-3
Some folks use an infrared thermometer directed on the thermostat housing. Others screw an aftermarket temp gauge sending unit into a water port on the engine somewhere. Whatever you do, the concept is to verify the existing sending unit and gauge with a secondary measurement system.