Can you put a humbucker into a single coil pickup position on a strat body? I might build my own guitar and would like an Alder body with a HSS set up (H in the bridge).

Is this possible or should I keep looking for a pre-made HSS alder body?

Yes it is possible. I know they make some humbuckers that are the same size as single coils or you could always "manipulate" the body to make a standard one fit.
Because deep down, I know you want to:

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Constructive criticism is always welcomed.
or use a router to make the H hole bigger before insertion...
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Give a little squeeze and say: "How do you do?"
If its a strat with a pickguard, you probably just have to cut a little bit off the pickguard, the body probably already has enough room for the humbucker.
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many/most strats are routed for at least HSS, assuming it has a pickguard. something like this:

of course a google search shows that a lot arent, so check before you buy something. of course if you are looking at one that isnt routed for HSS, you can either route it yourself or get some kind of stacked/mini humbucker.