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Because you're racing and because there's relatively little cushioning in XC spikes, you'll naturally land more on your forefoot (where the grip is as well) so you'll not overpronate in the same way (if at all).
If you're running slowly (!) and heel striking (and therefore overpronating), then there's not much point wearing a XC spike.
I'm a mild overpronator but still heel strike to some extent in XC. (I've seen the photographs!) In terms of spikes the best you can do is get a pair that have a little more cushioning than others, and I can recommend the Brooks Mach. (I've got the Mach 11 but they're recently brought out the Mach 12.) They're also a little wider than most other spikes I've tried on, so fairly stable.
If you're really heel striking it might be worth considering using trail shoes instead, which will have a more substantial heel. Don't know anything about those though.