Mark,
I changed my old wooden spreaders to a beautiful alumunum set made by JSI here in Tampa Bay a number of years ago.
Hopefully your mast is still stepped which will make things go easier.
Depending on the thickness of the aluminum, you can use sheet metal of tap threads for machine screws.
One thing to remember is that there is always a chemical reaction between dissimilar metals. Aluminum being the more noble metal in the galvanic scale, there is always some deteriation in the aluminum threads.
If you've ever removed threaded stainless threaded screws from aluminum which had been there for some time, you will notice a powder residue. That is the aluminum deterioration over time.
This doesn't have to be of concern; I just wanted you to be aware of such things.
Either method will work fine and have the strength to hold your burgees.
One suggestion is if tapping threads into the aluminum, use “thread lock” when attaching the screws. This will lesson galvanic corrosion over time and ensure the screws from coming loose.
Hope this was helpful.....
CR