Will a pickguard from an American Deluxe Strat fit a MIM standard Strat?

As far as body to neck fit, " Yes " but the screw holes may be a bit offset, something I have found to be occasionally the case, so if you only have to slant one or two screws very slightly it will be fine but if the holes are way out then you will have two options.
Fill in the old holes and once the new guard is placed where it should be mark out where the new holes need to be drilled.
The other option is to get a pickguard which is undrilled for screw holes and just place the old pickguard directly on top of the new one, secure with masking tape use the old guard as a template and guide to drill new holes.
You will have to countersink the new holes for the screws.

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"C'est parti mon Kiki "

Hey, thanks. I wasn't sure if the Deluxe body was maybe shaped a little different, thereby having a slightly different pickguard. Glad to hear that, if there is a difference, it'll be minor..

I just put an HSS loaded from a 2010 Am Deluxe on my 1998 MIM strat and everything fit perfectly. holes lined up, routing was fine. No problems at all.

Hope this helps.

Jeff