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The screw heads are bugle shaped, and sit flush to the surface of the aluminum block, if that helps.

Thanks in advance.

As others have pointed out, aluminum expands more with increasing temperature than steel does.

This means that the threads should be less tight when the entire assembly is at an elevated temperature and the screw is cooler by as much as possible.

After soaking the screws with penetrating oil at least overnight, I would then heat the entire assembly as high as the delicate parts will tolerate.

Then I would place a layer of styrofoam or other insulator over the screw, with a hole just the size of the screw head. Put the allen wrench into the screw head. Then spray with a can of freeze spray to cool the screw and the allen wrench as much as possible.

Then turn the allen wrench to remove the screw.

I once worked with a mechanical engineer who created alignment pins in an aluminum block by selecting just the right size stainless steel pins and drilled holes in the aluminum. He cooled the pins with liquid nitrogen and while holding the pin over the hole, he had an assistant drive the pin into the hole with a hammer. He assured me that the pin was NEVER going to come out or change position.

I presume that he sized the pins and the holes so that minimum distortion of the aluminum would take place as the pin warmed to the temperature of the aluminum.

That same engineer, on the same project, created a heating stage designed to operate at 300 degrees C out of a disk of aluminum inserted into a ring of stainless steel.

The idea was to have the increased thermal conductivity of the aluminum assure uniform temperature across the disk while the stainless steel, which has a lower thermal conductivity, would minimize the amount of waste heat needed to maintain temperature.

The alignment pins described above worked flawlessly. The heating stage was a total failure. Not enough room was allowed for the greater expansion of the aluminum under heat. The aluminum disk ended up squeezed by the stainless ring and developed a crown where there was supposed to be a flat surface.

Good luck with your project. Let us know what worked.

I have experienced this often while working on motorcycles.
The best results I have gotten is by using penetrating oil then using an impact driver with a heavy dead blow hammer.