my understanding is remove the weld from your existing steel botton press the stemm out. remove the stem from the new aluminum bottom (which is bigger around than your stem) machine a bushing to press into the aluminum triple that has the correct inside diameter to accept your steel stem. then weld it all up as I know nothing about welding steel and aluminum i would make sure whoever you have pressing and welding does. its a little more involved than finding some type of reducer ring.
hope this helps as les said leon has done this many times
