If you can afford it, buy the spare engine. At 150$ it's less than a new clutch basket I would think.
I don't know if the sequence of alu and steel really does matter. My guess is that it will work either way.
And yes, it may be possible to weld, depending on the alloy of the alu. Some alloys are easily weldable, other not so much. If you have a good aluminium welder in your vicinity. It has to be a person with the right skills, welding alu like this isn't for anyone. There may be a need for some milling / filing / turning to clean up the weld afterwards.
I think you could get away with just using it as it is, but, as has been mentioned, you may feel some vibrations.
A piece of advice from an old man: Finish what you have already started instead of starting on yet another modification. Get your bike running with the big wheels first. Then you can ride it and feel the joy of riding your own bike, the very bike that you have repaired yourself.
Later, build your SM wheels and experience the joy another time, as the bike is even closer to the bike of your dreams. This is how I do my bikes, one step at the time. Each modification brings me closer to the dream bike
And last but not least: Don't panic! No reason feel too stupid even when you break something. Of course it's not fun, but we have all been there. That is how you learn things. You will not do the same again
I did exactly the same on my Kawasaki Z650 when I was young. Luckily, I had a motorcycle breaker close by, and he had a spare engine in his barn. So I got a fairly cheap clutch basket and could go on riding.